Abstract: The nonionic waterborne blocked polyisocyanate(WBI) was prepared by reaction of isocyanate trimer and methoxy polyethylene glycol, and then blocking remnant active —NCO groups with blocking agent. Results showed that WBI emulsion with moderate viscosity and good storage stability could be prepared with low moleculare weight MPEG (Mn<2 000), and using MEKO as blocking agent. WBI emulsion prepared with IPDI trimer showed more uniform particle size distribution and better storage stability than that from HDI trimer. Physical properties of WBI emulsion changed dramatically once the MPEG content exceed an "critical value". The "critical value" varied for different kinds of ployisocyanate.