In consideration of the effects of the device instability induced by environmental fluctuation and the pollution of light-transmitting window on the on-line turbidity detection in line measurement, a new method to determine the turbidity in water is proposed based on the scattered-light power ratio measurement. This method uses two photodetectors to simultaneously detect the 90° scattered light power at different positions along the propagation path of a collimated light beam in the water environment. The turbidity of the water environment can be determined by calculating the logarithm of the scattered-light power ratio. The turbidity detector prepared based on this method detects 90° scattered light. The theoretical analysis indicates that this detector can in principle eliminate the influence of both the light source drifting and the window pollution on the measured turbidity. Experimental indiate the relationship between the two results measured with the turbidity detector and conventional transmission method.Furthermore,when the peak value of the light source fluctuates 30%,the turbidity coefficient changes only 5%,which indicates the detector is insensitive to the light drifting and window pollution.In conclusion, the detector is suitable for underwater detection of turbidity.