Injection seeding single frequency pulsed optical parametric oscillator (OPO) is an important technique to realize differential absorption lidar, whose spectral purity effects the energy absorption of laser pulses travelling via atmosphere directly and influences the accuracy of lidar system. Spectral purity demonstrates the ratio of seed mode energy to the whole pulse energy, which presents a comprehensive projection of spectral performances such as linewidth and frequency stability. In connection with 1.57 μm injection seeding OPO, factors influencing spectral purity are analyzed theoretically. A measuring system based on long path absorption cell is designed and built up. Experimental results show that spectral purity of injection seeding OPO can achieve 99.9% when seed injection power is 26 mW and OPO output energy is around 1.1 mJ.