Defects diagnosis of rolling element bearings operating under time-varying rotational speeds entails order tracking and analysis techniques that convert a vibration signal from the time domain to the angle domain to eliminate the effect of speed variations. Angular domain re-sampling will induce shock envelope deformation leading to errors of the analysis, and the effect is particularly noteworthy when the rate of speed change is significant. This paper presents a quantitative analysis of key factors affecting the accuracy of order analysis on rotating machines with varying speeds. An analytical model is established, simulated, and experimentally evaluated. Eliminating methods are proposed for which the angle value should be modified by removing the peak interval variations. The research findings show that deformation of the envelope in the width direction has no effect on results of the order analysis; interval variations of adjacent envelope peak have influence on the results of the order analysis; removing the peak interval variations in angle sequence can eliminate the effects of envelope deformation.