Improvement of measurement uncertainty evaluation of virtual instruments based on Monte Carlo method is proposed in tow espects. According to the mathematical modules, roundoff error module is built based on interval analysis method ,in order to evaluate the measurement uncertainty of virtual instruments caused by foundoff errors. The uncertainty evalution is effected by the quality of simulation,in order to reduced the impaction, an simulation quality testing and controlling module is built in the uncertainty simulation module. An example according to pressure measuring system was showed, and the result prove the improvement