The present invention relates to a device expanding the load range of a compression ignition gasoline engine and a method thereof. The device comprises a cylinder which is respectively connected with an air inlet channel and an exhaust channel by a vale, a piston which moves up and down in the cylinder and is provided with a drop pit at the top, an electric heater plug with a heating end extended into the drop pit when the piston runs to an upper stop point, and an oil atomizer with a nozzle opposite to the drop pit when the piston runs to the upper stop point. The method has the steps as follows: the oil atomizer sprays oil respectively during an air intake stroke and a compression stroke to form delaminated mixed gas in dense and thin areas; the electric heater plug heats the mixed gas in the cylinder to form heat delamination; the area with dense gas and high temperature in the drop pit of a combustion chamber fires firstly and ignites other areas to fire accordingly. The present invention controls the dense and the thin areas to fire successively, so as to reduce the pressure increase rate in the cylinder, ensure the reliable firing and achieve the purpose of developing HCCI combustion executable high and low charge ranges at the same time.