A spectral purity measuring system based on long path absorption cell is built to measure the spectral purity of 1572 nm laser. The error calculation formula of this system is deduced and the factors influencing the measurement accuracy is theoretically analyzed. The results show that the accurately measured spectral purity range is 90%-99.999%. Based on this system, the spectral purity of a self-developed 1572 nm injection-seeded optical parameter oscillator is measured to be (99.996±0.0005)%, which fulfills the industrial application requirements.