Abstract: A dynamic model for crop-stem lodging resistance was established, and a formula about critical force of crop-stem was extracted by using dynamic theory and method. This formula is suitable to several crops, such as maize, rice, wheat, and corn. According to this formula, a comprehensive evaluation regarding crop-stem lodging resistance was obtained. The results showed that the bigger the critical force, the stronger the crop-stem lodging resistance. Critical force is a current physics parameter to evaluate crop-stem lodging resistance. In the same kind of crop, if planting density or fertility of soil is different, the crop-stem lodging resistance will be different. The smaller the value of stem coefficient is, the stronger the crop-stem lodging resistance is. Stem coefficient is a physics parameter that evaluates crop-stem lodging resistance in the same kind of crop. The relationship between crop-stem lodging resistance and physics parameters of crop-stem was inferred. This can provide references for choosing variefies of crop, designing machines an comprehensively utilizing stem.