The invention discloses a performance evaluating method and system for high-voltage direct-current (DC) power transmission system 100Hz protection. The method comprises the following steps of: step 1, traversing and scanning asymmetric faults of an alternating-current (AC) electric network in an AC/DC interconnected network in which the high-voltage DC power transmission system stays, and determining an AC fault range influencing the DC 100Hz protection; step 2, determining a time boundary for the faults in the AC fault range, and also determining a delay constant value delta t of the DC 100Hz protection; and step 3, comparing the delay constant value delta t and an upper boundary t1, an upper boundary t2 and a shortest time t3, and evaluating the performance of DC100Hz protection. The system comprises a simulation module, a protection module and an evaluation module. The method and the system integrate the advantages of a short-circuit current calculation program and a PSCAD/EMTDC (power system computer aided design/electromagnetic transient simulation for DC) simulation computing program, fast recognition of the AC fault range influencing the DC 100Hz protection can be realized, and a simple, high-efficiency and practical engineering method is provided for evaluating the performance of DC100Hz protection.