Over the last years, the evolution of Vehicle-to-Everything (V2X) services have been prompted with the development of cellular technology. C-V2X standardize a distributed resource management that allows vehicles to autonomously select their transmission resource employing a Sensing Based Semi Persistent Scheduling (SB-SPS) protocol. The fundamental objective of the SB-SPS is to identify and periodic reserve a candidate group of sub-channels for the transmission of V2X. Nevertheless, the nature of the distributed sensing may give rise to severe transmission resource collision under time-varying vehicular environment, and there is still no clear mathematical expression for the reliability of the SPS scheduling. To this end, we try to provide a theoretical representation of the transmission collision problem. A mathematical definition of scheduling reliability is given based on the analysis of physical layer and MAC layer of SB-SPS, which reveals that resource conflict Probability increase as the resource selection window overlap ratio and channel busy ratio increase. Then we provide modifications for SPS scheduling enhancement by adding some processes to the resource reselection step of SPS, which is demonstrated effectively prevent vehicles occupying non-conflicted resources, and also allows vehicles adaptive scheduling to select with other vehicles according to their resource reselection. Simulation results demonstrate the theoretical representation of reliability, compared with the standardized SPS scheme, our proposed scheme achieve performance improvement in terms of the packet delivery ratio, and suitable for practical vehicle density, moving speed, and different traffic size.