In this article, N-hydroxyphthalimide (NHPH) has been prepared with phthalic anhydride and hydroxylamine hydrochloride. The blocked toluene diisocyanate (NTDI) by NHPH was synthesized also. The effects on the synthesis of NTDI were studied, and the deblocking temperature of NTDI was read by TG-DSC and IR. In the meantime, actual crosslinking reaction conditions on the systems of polyhydroxy compounds were surveyed also. The stability of the mixture of NTDI and polyhydroxy compounds was observed by determining the viscosity of it.