Current transpiration models have commonly used the greenhouse climate as a boundary condition. The measured transpiration, the water holding capacity or the accumulated solar radiation was normally taken as the baseline to determine the irrigation amount of greenhouse crops. But, the determination of the irrigation amount required many monitoring parameters, and the accuracy of the monitoring instrument was relatively high. In this study, the optimal irrigation level was determined for the potted tomatoes with coconut bran in a solar greenhouse using the evaporation of a 20 cm evaporation pan as the baseline. The evaporation pan coefficient and irrigation levels were set in the three growth periods (seedling stage: 0.2 (ET1), 0.4 (ET2), and 0.6 (ET3); flowering and fruit setting stage: 0.3 (ET1), 0.5 (ET2), and 0.7 (ET3); fruiting stage: 0.7 (ET1), 0.9 (ET2), and 1.1 (ET3)). A comprehensive evaluation was made to clarify the effects of the irrigation amount on the tomato yield per plant, Water Use Efficiency (WUE), and fruit quality. A transpiration model was established for the potted tomato grown in the coconut bran in a solar greenhouse using the optimal irrigation water level. Two field tests were conducted to verify the model. The results showed that the nitrate and lycopene contents of ET1 treatment were significantly higher than those of other treatments by 18.99%-59.19% and 15.47%-30.16%, respectively. The plant height, soluble sugar, and soluble protein contents of ET2 treatment were significantly higher than those of other treatments by 8.54%-14.27%, 28.61%-32.99%, and 38.70%-70.83%, respectively. The growth stage transpiration per plant, the average daily transpiration per plant, the tomato yield per plant, and the total water consumption of ET3 treatment were significantly higher than those of other treatments by 8.95%-43.49%, 15.08%-33.12%, 2.50%-19.00%, and 11.71%-18.02%, respectively. The ET2 treatment improved the WUE by 10.05% and saved the irrigation amount by 22.23% with only a 2.50% reduction in the yield per plant, compared with the ET3 treatment. A principal component analysis was carried out on the tomato yield per plant, WUE, and fruit quality. The highest comprehensive score of the two principal components was the ET2 treatment, indicating that the ET2 was treated as the optimal irrigation amount of tomato potted with coconut bran in the solar greenhouse. Path analysis and correlation analysis were performed on the indoor daily cumulative net radiation (M), daily average temperature (T), daily average relative humidity (RH), tomato Leaf Area Index (Leaf Area Index, LAI), and daily transpiration. According to the correlation analysis, the degree of influence of each factor on the daily transpiration was ranked in the descending order of M, T, and LAI, indicating the very significant positive correlation of daily transpiration with M, T, and LAI. According to the path analysis, there were direct effects of M and LAI on the daily transpiration. By contrast, there were the indirect effects of T on the daily transpiration, where the M promoted a positive effect on the daily transpiration. Mcquard method was also used to estimate the daily transpiration, M, T, and LAI in the ET2 treatment, according to the correlation trend of daily transpiration factors. The determination coefficient was 0.896, indicating a well fitted model. The daily transpiration and M, while the T and LAI of the other two treatments were selected to test the transpiration model. The determination coefficient was 0.851, while the Root Mean Square Error (RMSE) was 49.88 g, and the relative error was 11.88%. Consequently, the transpiration model of ET2 treatment can also provide the scientific basis and decision-making reference for the efficient production and intelligent irrigation of potted tomatoes with coconut bran in the solar greenhouse.