Abstract: This paper proposes an improved decision-making method based on deep reinforcement learning to address on-ramp merging challenges in highway autonomous driving. A novel safety indicator, time difference to merging (TDTM), is introduced, which is used in conjunction with the classic time to collision (TTC) indicator to evaluate driving safety and assist the merging vehicle in finding a suitable gap in traffic, thereby enhancing driving safety. The training of an autonomous driving agent is performed using the Deep Deterministic Policy Gradient (DDPG) algorithm. An action-masking mechanism is deployed to prevent unsafe actions during the policy exploration phase. The proposed DDPG?+?TDTM?+?TTC solution is tested in on-ramp merging scenarios with different driving speeds in SUMO and achieves a success rate of 99.96% without significantly impacting traffic efficiency on the main road. The results demonstrate that DDPG?+?TDTM?+?TTC achieved a higher on-ramp merging success rate of 99.96% compared to DDPG?+?TTC and DDPG.