Using Li-cor's plant canopy analyzer, taking the corn and wheat as examples, the accumulated LAI from the top of the canopy to the different levels of the colony were measured and analyzed to determine the height of the active surface of crop colonies. The data of measured LAI were simulated according to the models given by Raymond Pearl (1870-1940) and Gompertz. The results show that the two models are suitable for the simulation of the LAI in the crop colony and the correlation coefficients are above 0.95. Raymond Pearl's model is better than Gompertz's for the wheat colony, while the latter is more suitable for the corn canopy and the correlation coefficient is up to 0.99. The height of active surface of wheat colony is quite different from 2/3 of the plant height, which is used to determine the active surface roughly. Meanwhile the height of active surface in the corn colony is similar to the 2/3 plant height, the difference between them is only 11 to 17 cm. So using the 2/3 of the plant height to estimate the active surface height is rational but is likely to produce obvious errors to the wheat canopy.