On the basis of mixer circuit model of light, the terahertz power generated by the carbon nanotubes (CNT) photomixer is analyzed. By simulating mixer conductance, impedance of the antenna, and light plus paranoid voltage, it is shown that the improved mixer conductance, antenna impedance and light plus paranoid voltage can improve the output power of terahertz waves. The output power can reach dozens of microwatt level in the small-signal limit.