A design of ratio turbidity sensor fabricated by laser refractive index modulation is proposed and tested. Both waveguide theory and detecting knowledge are combined. The turbidity sensing structure is made from UV curable optical polymer and a special “airing dry method” is used to dispose the unexposed part. The whole progress only takes 7~8 minutes and thus this method can greatly promote fabrication efficiency. Experimental test is conducted by using standard turbidity solutions, and the related data are processed with a SISO algorithm neural network system. The results curves are obtained for scattering coefficient and turbidity of the standards,which proves that the sensor system has a relative error rate of 5%.