The vertical cavity surface-emitting laser(VCSEL) of the chip scale atomic clock(CSAC) generates a coherent laser with a specific optical frequency difference through the microwave modulation.After the atoms reacting with light beam,the frequency of the transition spectrum line can be used as the reference frequency standard,and the output of high precision frequency signal can be obtained.Therefore,vertical cavity surface-emitting lasers are very important in the CPT atomic clock system.The internal modulation principle of VCSEL laser is introduced.The experimental test platform for its internal modulation characteristics is built.The response characteristic of the laser to the radio-frequency modulation is studied.The change of the laser sideband signal with the injection current and the radio-frequency output power is recorded.The influence of the radio-frequency modulation on the laser sideband signal is analyzed and the sideband asymmetry caused by the Bogatov phenomenon is present.The experimental results show that the optimized high frequency modulation spectrum can be obtained when the frequency of the radio-frequency signal is 3.41734 GHz,the injection current is 1.2 mA and the RF power is 3.5 dBm,which provides the high quality light source for the chip scale atomic clock.