Turbidity meter is a crucial instrument to evaluate the physical property of water. The authors and their team work on studying the measuring principle of turbidity, designing the optics and electronic structure, and finally fabrication a real instrument. This instrument based on the ATmega128 microcontroller, and also have several other modules: LED driver, analogdigital converter, current voltage converter, isolator and etc. Mixed with turbidity standard substance and ultrapure water, 17 samples were made uniformlyspaced from 0 NTU to 400 NTU. The experimental result shows that scattered light intensity and transmission light intensity is highly consistent with the equation, the linearity of the curve is very good and the experiments meet with the anticipated goal.