Abstract: A neutron and core thermal dynamic model is built to develop a computer calculation code CICC to simulate the transient behavior of core power. Simulation results show that the power control system works well when k_eff locates in 0.94 and 0.98 and serious power overshoot will appear when k_eff ＞ 0.98. Control effects could be largely improved after introducing a feed-forward signal into the PID controller, which could effectively eliminate the reactivity disturbance lower than 1×10^-3. The results also suggest that the reactivity will oscillate divergently if control rods in core when there is accelerator flow loss accident. Calculation result points out that the optimal match of Q and k_eff locates in the interval of（0.94, 0.96） and（1.2×10¹⁰ cm^-3 s^-1, 1.85×10¹⁰cm^-3 s^-1） respectively.