In this paper, we study the road safety message transmission problem under the hybrid network architecture, where the centralized network and the distributed network coexist and we propose the time division multiple access protocol under the hybrid network architecture. In order to solve the problem of collisions between nodes in different working modes, we define the mapping rules between virtual frames and physical frames. Through an iterative algorithm, we estimate the node arrival probability and the number of competing nodes, and design a frame structure with a variable frame length, which can significantly lower the access delay. The simulation results show the accuracy of the estimation algorithm, and the proposed protocol has good performance in access delay and access failure probability.