The biomass of Monascus purpurcus solid-state fermentation influenced by low frequency alternating magnetic field (50 Hz, 0~2 mT) was main studied in order to discuss the magnetic field influence on the performance of engineering bacteria fermentation. Results showed that the magnetic treatment could promote Monascus purpurcus biomass producing in the third day with the maximum improving of 29.9% at 0.6 mT. The biomasses of treatment group and control group reach maximum in the eleventh day with the whole procedure of 15 days and magnetic field intensity of 0~2 mT. But the biomass and substrate consumption ratio of treatment group and control group did not show obviously difference. Low-frequency alternating magnetic field could promote Monascus purpurcus growth, then change their metabolic way and enhance the fermentation benefits. 