Abstract: Lycium Barbarum (also called wolfberry) is a deciduous shrub mainly grown in northwestern China. The flowering phase of Lycium Barbarum is from April to June when the frost occurred frequently. The harm of frost at this time can directly affect the yield and quality of Lycium Barbarum. It is necessary to study the frost index of Lycium Barbarum in the flowering phase for frost forecast, early warning, and evaluation. Taking the species of 'Ningqi No.1' as research object, we investigated the effect of low temperature and its duration on the frost index of the Lycium Barbarum flowering phase. In the spring of 2016-2019, an artificial frost test box was used to conduct the frost index experiments in Yinchuan, Ningxia Province, northwestern China. An invitro test was conducted in the low-temperature treatment. The young branches in a healthy growth state were selected from the experimental field in the Ningxia Lycium Barbarum Research Institute. The experimental temperature gradients were set from -9℃ to -1℃, with an interval of 1℃. The selection of temperature gradients referred to the temperature reduction during the spring frost disasters in Ningxia from 2010 to 2018, and the observation records of the Lycium Barbarum agro-meteorological disasters. Different duration, from 1h to 6h, was set at an experimental temperature in each experiment. Three samples were used at each experimental temperature for reproducibility. More than 50 flower buds were selected for the early flowering and full flowering periods in each replicate. The room temperature was used as the initial temperature, and it was reduced to 0°C at a rate of 4.0°C/h, finally decreased to the experiment temperature at a rate of 1.0-3.0°C/h. The experiment temperature can be holding for the preset duration. The experiment box was closed once the experiment was completed, where the temperature naturally rose to room temperature. The freezing conditions of Lycium Barbarum flowers can be examined after 24 hours. In the post processing step, a concept of harmful accumulated temperature was introduced to systematically elucidate the effect of low temperature and its duration on Lycium Barbarum flowers. The results showed that the freezing rate and freezing degree cannot increase as the decrease of temperature and duration below the critical low temperature. A approximate formula of harmful accumulated temperature was modified using a piecewise function, in order to describe the degree of frost damage of Lycium Barbarum flowers at different temperature stages, according to the actual situation of damage. Then, combined with different freezing rates, the frost index of the Lycium Barbarum flowering phase was established based on the harmful accumulated temperature, and verified by the field experiments. The experimental results showed that when the temperature dropped to -1℃, there was no sign of freezing on the flower organs of Lycium Barbarum, whereas, when it fell to -2℃, the signs of freezing began to occur, with the freezing rate up to 100% at -8℃, indicating that the temperature range of frost damage was -8--1℃ during the Lycium Barbarum flowering phase. In addition, the different frost resistance of flowers varied at different stages. Specifically, flower buds showed the strongest frost resistance, followed by that in the early flowering period, and the weakest in the full flowering period. The proposed frost index based on the harmful accumulated temperature can be used to determine the critical low temperature of different forms of flower organs caused by frost, particularly considering concurrently the low-temperature intensity and duration, thereby to precisely predict the freezing conditions of the Lycium Barbarum flowers. The finding can provide a promising approach for the accurate forecast, early warning, and assessment for the frost of Lycium Barbarum.