The invention belongs to the pressurized water reactor overall structure design technology, and specifically discloses an integrated small reactor. The integrated small reactor comprises a pressure vessel, and a reactor core, a compressing assembly, a control rod assembly, a guide assembly, a control rod driving mechanism and a once-through steam generator which are arranged inside the pressure vessel, and further comprises a main pump directly connected with a cylindrical body of the pressure vessel, wherein the reactor core comprises a reactor core barrel and a fuel assembly; the compressing assembly and the control rod assembly are arranged inside the reactor core barrel; and the pressure vessel is directly connected with the main pump, the main pump is horizontally installed, and an inlet thereof is provided with a flow distribution device. Heating equipment is arranged on a top cover of the pressure vessel, a coolant inside the pressure vessel is subjected to pressure control directly through temperature adjustment, that is, main equipment such as the steam generator, voltage-stabilizing equipment and the control rod driving mechanism are arranged inside the pressure vessel of the reactor at the same time. According to the integrated small reactor, the external loop pipeline is omitted, large break accidents are eliminated, and the structure is compact; and the control rod driving mechanism is arranged internally, the rod ejection accident is eliminated, and the fixation safety is greatly improved.