@Lishuo_Pan You can arm the UAV using the mavros service /mavros/cmd/arming
Make sure you publish enough position setpoint messages at a high rate to be able to switch to offboard board and arm using this service. This approach works for me. You may have to call this service again if it doesn’t arm the UAV. You could do this by checking the arm status. Usually, in 2-3 attempts, it arms for me.