This paper mainly studies the bias control performance for multiple bias voltages of 16-quadrature amplitude modulation 16-QAM optical transmitter based on IQ optical modulator. The cooperative control of two Mach-Zehnder modulators′ (MZMs′) bias voltages and Mach-Zehnder interferometer′s (MZI′s) bias voltage is analyzed theoretically. Multiple extreme values of the average output optical power as the monitoring signal of feedback control for two MZMs′ bias voltages are also studied by numerical simulation. The results show that the average optical power of 16-QAM optical transmitter is the extreme value when MZMs′ bias voltages are biased at the null transmission points. This conclusion is unaffected by MZI′s bias voltage, and there does not exist the risk of trapping in local extremum if the amplitude of the driver signal is controlled in the proper range.