An experimental scheme is proposed and realized, with which the characteristic of a vertical-cavity surface-emitting laser(VCSEL), a key component of a passive coherent population trapping(CPT) atomic clock, is studied through atomic absorption spectra toward laser. By switching on and off the input microwave for modulating VCSEL, atomic absorption spectra toward monochromatic and multi-chromatic laser are alternatively recorded. The characteristics of VCSEL, including temperature response rate, temperature change along microwave power, wavelength response to its temperature, and the CPT state preparing efficiency are studied with the spectra measurement. This scheme provides a convenient experimental method to measure and study VCSEL, and a useful tool for developing CPT atomic clocks.