Abstract: By comparing the measured and model calculated P absorbed by seedlings, this paper dealt with the predicting ability and its influencing factors of selected mechanistic models of nutrient uptake by both maize and soybean seedlings. The results showed that: (a) All of the three kinds of selected models can reflect the changing trend of P uptake by seedlings of maize and soybean; (b) Among the three selected models, the predicting ability of SC Model is the poorest and that of C2 Model including the effect of root hairs is the strongest; (c) The important factors influencing models' predicting ability are soil nutrient and water levels, soil N forms supplied soil properties and soil Ca levels or pH value; (d) The soil conditions that with P and water stress, clay texture, applied NH4+-N and applied limestone in acid soil can make it become poor for the mechanistic model based on numeric solution to predict P uptake, and this is because that the above soil conditions can induce or strengthen the other actively adaptive mechanism of seedlings' root system to intake P which has no been considered in the models.