Vehicle Ad-Hoc networks (VANETs) needs to provide time-delay sensitive communication service for security application and non-security application, which makes medium access control (MAC) not only meet the time delay and reliability of security information, but also ensure the service channel utilization of non-security information, and also proposes challenges for the design of MAC. Considering about the requirements of security application and non-security application of VANETs for information transmission, MAC improvement algorithm I-TDMA-MAC based on TDMA has been proposed. I-TDMA-MAC considers about both the time delay and reliability of security information transmission and the utilization of service channel. I-TDMA-MAC allocates two sub-slots for reach vehicle, improves security information transmission performance; at the same time, re-allocates idle slot and improves utilization of service channel. Simulation results have shown that compared with TCMAC, the transmission rate of data packet of I-TDMA-MAC has been increased to 99% and the utilization of service channel has been increased to 90%.