Abstract: On the basis of theoretical analysis, the discrete model, differential equations of motion and computational formular of critical rotation rate of chopping drum of field straw chopper are established .what's more .computer programmes are worked out in this paper. Using the programmes, the critical rotation rate of chopping drum of field straw chopper JH - 100,IHF- 150,JHF- 130 are calculated particularly and main factors,influencing critical rotation rate are analysed.