In order to solve the problem that the performance of impedance matching deteriorates due to the insufficient precision of the inductor in miniaturized antenna impedance matching, a complementary metal-oxide-semiconductor（CMOS） circuit for automatic antenna impedance matching is proposed. Firstly, a new impedance-matching method is proposed, which realizes impedance matching by adding capacitors to the input and end of the antenna. Then by integrating the matched capacitors, an automatic impedance matching circuit is proposed. The main principle is to judge the input power of the antenna through the power detection circuit and the peak detection circuit and to change the input impedance of the antenna through the capacitor scanning circuit and the capacitor controlled by the program. The innovation of the circuit is also reflected in removing the step of adjusting impedance matching using a network analyzer, so as to reduce the influence of test environment change on impedance matching. The design requirements of the output impedance of the RF power amplifier are reduced. The precision of impedance matching can be changed by designing the precision of matching capacitors.