Dynamic, static and metadynamic recrystallization of as-cast P91 alloy steel are investigated through single and double-hit unidirectional thermal compression experiments under different deformation temperatures,strain rates and deformation degrees. The kinetics of static and metadynamic recrystallization are established and the results show that,the static recrystallization fraction is increasing with the increase of deformation temperature,strain rate and deformation degree; the metadynamic recrystallization fraction has the monotone increasing relationship with deformation temperature,strain rate and deformation degree. The softening methods of the deformed as-cast P91 alloy can be divided into three conditions through the values of true strain: whenε<ε_c,the static recrystallization operates as the prime softening mechanism in the inter-pass time; whenε_c<ε<ε_T,both static and metadynamic recrystallization are involved in the softening; whenε>ε_T,the metadynamic recrystallization is the predominante softening mechanism. In addition,the microstructure of the specimens are observed,it can be found that the grain sizes of MDRX are smaller than SRX in the same deformation conditions,the grain sizes of recrystallization are increasing with the increase of deformation temperature and decrease with the increase of strain rate.