The invention discloses a preparation method of a heat-resistant modifier N-tert-butyl maleimide. The preparation method comprises the following steps: firstly, adding acetone and maleic anhydride into a reaction container, and stirring for dissolving; then dropwise adding mixed liquid of tert-butylamine and acetone, and heating for reaction to obtain N-tert-butyl maleamic acid; heating an obtained reaction solution, and adding a polymerization inhibitor and a sulfoxide chloride solution for reaction; after reaction, heating an obtained reaction solution, slowly dropwise adding N,N-dimethylaniline, and after dropwise adding, preserving heat for reaction; after reaction, adding warm water into an obtained reaction product respectively for performing liquid separation for three times; heating for concentrating liquid obtained after liquid separation, recycling an acetone solvent, and collecting a fraction, namely the product N-tert-butyl maleimide. According to the preparation method, the reaction operation is convenient, used raw materials are simple and easy to obtain, the product quality is stable, and the industrial production is easy; when the preparation method provided by theinvention is used for preparing a target product, the overall cost is significantly reduced and a large amount of waste liquid is avoided; therefore, the preparation method has significant economic and social benefits.