To improve the bonding efficiency of microfluidic chips, an in-mold bonding method is researched by taking a Polymethyl Methacrylate(PMMA) microfluidic chip as an research object and a micro injection machine as a working platform. Clamping forces and bonding temperatures are provided by the micro injection machine and a mold temperature machine. Isopropanol is selected as assistant solvent to reduce the bonding temperature and bonding pressure with its solubility. At 30 ℃ to 70 ℃，a microscope and a step profiler are used to analyze the influence of assistant solvent on surface morphologies and micro channel structures of chips at different temperatures. An in-mold bonding experiment is operated with the assistant solvent, then the electric universal testing machine is utilized to measure the bonding strength and to optimize the process parameters. Experimental results illustrate that the influence of isopropanol on bonding quality is related with the bonding temperature and bonding period. The surfaces of chips chap and the micro grooves become deformed or block at higher temperatures. The surface quality and morphologies of micro grooves are correspondingly improved when the bonding temperature is 35 ℃, the bonding time is 5 min, and the bonding strength is no less than 2.64 MPa.