A bias point feedback control system of Mach-Zehnder modulator (MZM) for transferring one pulse per second (1PPS) is used in time and frequency transfer via optical fiber. The low-level voltage of the 1PPS output by photodiode is used to measure the drift of the bias point, which is set between minimum point (Null) and orthogonal point of positive slope (Quad+) on transmission curve. The measured signal is digitally processed and used to control the feedback system to stabilize the bias point. The theoretical derivation of the principle is described and the performance of the system is compared with that of commercial bias point stabilized system based on dither method. The experimental results show that the behavior of our system is better, and the instability results from the dither can be avoided. The peak-peak fluctuated value of the propagation delay of the 1PPS is 174 ps, and the root-mean-square (RMS) value is 18 ps. The time deviation (TDEV) of the 1PPS for the averaging time of 104 s decreases to 1.7 ps.