In present work the stress rupture property and related microstructure evolution of FB2 rotor steel are investigated by metallographic analysis, scanning electron microscope and transmission electron microscope. The investigation shows that the microstructure of FB2 rotor steel is composed of tempered martensite. Large amount of fine M23C6 carbides precipitates on original austenite grain boundary and in matrix. Decomposition and broaden of martensite lath can be found with prolonging stress rupture duration. The dislocation density decreases at the same time. Meanwhile, the M23C6 carbides located at grain boundary and martensite lath boundary grows. Laves phase gradually grows at original austenite grain boundary and aggregates into a chain distribution. The microstructure evolution at high temperature can apparently influence high temperature stress rupture property. While the FB2 roto steel can still meet the requirement that the stress is higher than 100 MPa after 630℃stress rupture test for 105h.