A runtime assurance-detect and avoid module (RTA-DAAM) is proposed to address the avoidance issue of unmanned aerial vehicles in the urban air mobility (UAM) scenario. It does this by identifying external environmental information sources and providing control commands and alarm information to guarantee the effectiveness and safety of aircraft in the UAM scenario. The control methods of advanced and standby collision avoidance modules in RTA-DAAM are designed. Considering the importance of coordination of detection, decision-making and warning functions, defining the aircraft’s own clearance boundaries, prevention boundaries, and obstacle safety boundaries, and dividing the different aircraft states so as to complete the design of the decision-making and warning logics. The simulation results show that RTA-DAAM can ensure the desired separation distance of the aircraft at a loss of only 3.40% of efficiency, so as to ensure the safety of the DAA system. The implementation of RTA technology can increase UAV safety time to 96.9%, providing UAV safety to the greatest extent possible, according to the real flight results of tiny UAVs.