Future cross-domain aircraft will face multiple atmospheric reentry processes. During hypersonic flight through the atmosphere， the vehicle is enveloped by a plasma sheath， and the “radio blackout” caused by this sheath will continue to hinder reliable information transmission. The plasma sheath channel exhibits fast time-varying characteristics， which result in communication symbols experiencing random deep fades， making it challenging for the receiver to track the channel coefficients in real-time. This presents a significant challenge to the reliable transmission of telemetry and control communication systems. To address these issues， this paper investigates the coherence time of the plasma sheath channel’s time-varying characteristics. The paper first analyzes the factors contributing to the dynamic characteristics of the plasma sheath channel and establishes a dynamic plasma sheath channel model. Then， a method for calculating the channel coherence time based on the received signal envelope is designed， followed by simulation analysis and experimental validation. The results show that the channel coherence time is inversely proportional to the plasma excitation frequency and exhibits a trend of decreasing and then increasing as the carrier frequency increases. By adjusting the symbol rate to match the coherence time， an appropriate symbol rate can be set based on the calculated coherence time under an electron density of 1×10¹⁸ m^-3， resulting in a two-order-of-magnitude improvement in bit error rate performance. The calculation of the plasma sheath channel coherence time can provide valuable references for future communication scheme designs.