Nonlinear dynamics of semiconductor lasers under optical injection can exhibit intensity oscillations at tunable microwave frequencies. Based on the rate equation model, the system under periodic oscillation is analytically investigated. By considering two dominant co-existing optical frequency components, analytical expressions that relate the injection strength and the injection detuning frequency to the microwave frequency, optical power, and microwave power are yielded. The analytical results are in excellent agreement with the simulation results of the well-established rate equations. The analysis shows that, when the Nonlinear dynamics of semiconductor lasers under optical injection can exhibit intensity oscillations at tunable microwave frequencies. Based on the rate equation model, the system under periodic oscillation is analytically investigated. By considering two dominant co-existing optical frequency components, analytical expressions that relate the injection strength and the injection detuning frequency to the microwave frequency, optical power, and microwave power are yielded. The analytical results are in excellent agreement with the simulation results of the well-established rate equations. The analysis shows that, when the linewidth enhancement factor is much larger than unity, the oscillation can be explained as the result of beating of the optical injection frequency and the red-shifted cavity resonance frequency. The red-shift is due to the reduction of the average charge carrier density and the corresponding antiguidance effect. The results are useful for applying the system to photonic microwave generation,width enhancement factor is much larger than unity, the oscillation can be explained as the result of beating of the optical injection frequency and the red-shifted cavity resonance frequency. The red-shift is due to the reduction of the average charge carrier density and the corresponding antiguidance effect. The results are useful for applying the system to photonic microwave generation.