In a hierarchically connected UHVDC system, there is a delay in the start time of the non-fault layer commutation failure prevention (CFPREV). This makes it unable to respond to the severity of the fault in time to output an appropriate trigger angle advance. This may lead to an increased risk of commutation failure of the high and low converters at the same time. Thus a CFPREV coordination strategy for high and low converters based on the feedforward of the trigger angle advance of the fault layer is proposed. This strategy uses the characteristics of fault converters that can more sensitively respond to the severity of faults, and it introduces the trigger angle advance of the CFPREV of the faulty layer into the CFPREV of the non-faulty layer, so that the start time of the CFPREV of the non-faulty layer is advanced. In addition, the strategy also introduces a DC current coordination coefficient to obtain a more appropriate trigger angle advance. A simulation model is built in PSCAD/EMTDC to verify the proposed control strategy under different working conditions. The results show that this strategy can quickly respond to inverter-side AC system faults, and reduce the risk of simultaneous commutation failures of high and low converters. The proposed control strategy plays a role in solving the problem of simultaneous commutation failure of high and low converters in the hierarchically connected system. This work is supported by the Science and Technology Program of Sichuan Province (No. 2021YFG0026).