This paper describes a poly(dimethylsiloxane) (PDMS)-based microfluidic platform for constructing the phase diagram of poly (N-isopropyl acrylamide) (PNIPAM) in aqueous solution. The PNIPAM solution was delivered into a nanoliter chamber through the main microchannel. An osmotic pressure difference was established between the chamber and the control microchannel by flowing a high-concentration salt solution in the control microchannel. Controlled evaporation of water resulted in increasing concentration of PNIPAM. A phase diagram of PNIPAM was built by measuring the cloud points at different concentrations, with a minimum point at approximate to 40 wt.-%. The microfluidic platform has the advantages of low sample consumption and rapid heat exchange rates, and allows studying viscous polymer solution.