The principles of three common measuring methods of the water turbidity, scattering method, transmission method and ratio method are introduced. Combined with pulse width modulation (PWM) and demodulation technique, the method for turbidity measurement using light intensity modulation is proposed and the experimental system and the realizing circuit of modulation and demodulation is described in detail. The experimental system is used to measure the turbidity of Formazine standard solution of different turbidity values. The measured data is listed, then the turbidity values fitting curve using the three methods are drawn. Through the analysis and comparison of the experimental results, it’s showed that when using the ratio method combined with the modulation technique, the interference of background light can be eliminated effectively, so the measuring range of the water turbidity can be improved with high measuring accuracy.