High temperature shape memory polymers that can withstand the harsh temperatures for durable applications are synthesized, and the aromatic polyimide chains with flexible linkages within the backbone act as reversible phase. High molecular weight (Mn) is demanded to form physical crosslinks as fixed phase of thermoplastic shape memory polyimide, and the relationship between Mn and glass transition temperature (Tg) is explored. Thermoset shape memory polyimide shows higher Tg and storage modulus, better shape fixity than thermoplastic counterpart due to the low-density covalent crosslinking, and the influence of crosslinking on physical properties are studied. The mechanism of high temperature shape memory effects based on chain flexibility, molecular weight and crosslink density is proposed. Exposure to thermal cycling from +150 °C to -150 °C for 200 h produces negligible effect on the properties of the shape memory polyimide, and the possible mechanism of high and low temperature resistant property is discussed.