It is necessary to implement pretreatment operations for a paper based microfluidic device. A paper based microfluidic device with SAW driving microfluid has been implemented. Trance analyte to be detected was absorbed into a filter paper at first, and mounted on PDMS blocks on a piezoelectric substrate to ensure that the indicting filter paper has a little gap with the piezoelectric substrate. Reagents were then pipetted on the piezoelectric substrate and transported by surface acoustic wave excited by an interdigital transducer, which was fabricated on a 1280-yx LiNbO3 using micro-electrical technology. A color was developed due to chemical reaction, and the analyte concentration was evaluated by its grey value. Nitrate ion was detected using the microfluidic system.