With the development of next fifth generation (5G) communication system, unlicensed spectrum has been considered as a central candidate to offload data traffic. In addition to the most important issue of fair coexistence with Wi-Fi, another problem is to reduce the transmission latency in unlicensed band. Grant-free can be used as an important solution to reduce latency by randomly accessing without scheduling, but it brings lower reliability. In this paper, the grant-free uplink transmission with multiple transmission times (GUL-M) is introduced into the coexistence system of Wi-Fi and new radio in unlicensed band (NR-U) to improve transmission reliability by transmitting one data packet several times at the same time. Based on Markov chain model, the performance of coexistence system is analyzed by deriving the transmission probability and collision probability of Wi-Fi and NR-U with GUL-M. Corroborated by simulations, our approach can achieve the tradeoff between reliability and channel utilization efficiency by dynamically adjusting the transmission times, which shows that as the transmission times increases, there is a maximum throughput efficiency for both NR-U with GUL-M and the coexistence system.