Trimethylolpropane maleates(TMPMs) are prepared by esterification from trimethylolpropane(TMP) and maleic anhydride(MA). The effects of feed ratio, temperature, reaction time and catalyst are studied, the results indicated: esterification rate of MA can reach 100% with n(TMP)/(MA)=1:2 under 90℃ during 120min, esterification rate of MA can reach 100% with n(TMP)/(MA)=1:2.5 under 90℃ during 240min, for n(TMP)/(MA)=1:3, esterification rate of MA reach 90.7% under 90℃ during 240min and will not change any more. Catalyst addition is helpful for increase of esterification rate of MA, which can reach over 99% without diesterification of MA. The structure and composition of TMPMs are characterized and analysed by NNM R, the Average Double Bond Functionality(ADF) of TMPMs are calculated by esterification rate.