To provide theoretical bases for developing thermal storage boiler(TSB), research isdone on the inner proceas of TSB. According to energy conservation law and massconservation law, two groups of quations are established to describe the dynamic property ofTSB. Also detailed discussions are made on the operation efficiency, the safety of economizerand the reliability of water circulation of TSB when the pressure of TSB is varying. In addi-tion, advices on proper output range and pressure range for designing a coal- fueled varyingpressure TSB are suggestal. To state the dynamic property of TSB, numerical calculations aremade on the working example.