For Mg-Y-Nd-Zr-Gd alloy, a compression deformation test was conducted under conventional strain rate(1×10-3- 1 s-1) and ultrahigh strain rate(>1×103 s-1) at room temperature, and its mechanical response and microstructure evolution were studied comprehensively. The results show that the deformation characteristics of Mg-Y-Nd-Zr-Gd alloy under two strain rate ranges are different. Within the conventional strain rate range, the mechanical properties of the material are similar under different strain rates, and the work hardening rate of the material continues to decrease after yielding. However, when entering the ultrahigh strain rate range, the yield point of the material rises, and the work hardening rate appears to plateau and continues until the material breaks. Under conventional strain rate conditions, the dominant deformation mechanisms are basal slip and tensile twins, while a large number of secondary twins appear after the ultrahigh strain rate deformation. According to IGMA analysis, it is found that the non-basal <c+a> slip systems are activated at the ultrahigh strain rate. The activation of slip systems with high CRSS value and the interactions between secondary twins and complex non-basal slip are the main reasons for the yield point increase and abnormal work hardening of Mg-Y-Nd-Zr-Gd alloy at ultra high strain rate.