Driving a copper projectile to impact a cryo-target made of aluminous alloy at the speed of 2.21 km·s-1 with a two stage light gas gun, a proper one dimensional shockwave with a speed of 18.76 km·s-1 was generated and directly acted on a uniform liquid in target, which was condensed by proportional gaseous carbon monoxide and nitrogen. At the measurement of Hugoniots, the full linear spectrum entirely dissociated with the plasma under the shock pressure of 33.5 GPa was caught by the use of intensified charge coupled device and accurate spectrographic technology. From the analysis of the spectrum, the authors know that the transient spectrograph with six channels can be used to measure and record the course of shock compression-irradiancy reliably, and the emitted spectral lines of shock compressed products indicated that the thermal dissociation and phase transition had occurred in homogeneous liquid of CO-N2. Furthermore, comparing the spectral intensity of lower central wavelength with that of higher, the fact of stronger intensity of 488 nm also validates that changes from “optic thin” to “optic thick” exist indeed in dense hydrocarbon liquid acted on by shock pressure.