The disclosure provides a heterojunction bipolar transistor and a preparation method thereof. Since an emitter region has the same physical structure as a base region, and improves frequency characteristics of the device; Simultaneously with biaxial strain, uniaxial strain is introduced. Carrier transmission time in the collector region will be effectively reduced. By this structure, the width of the effective collector region is reduced, the collector junction capacitance is reduced, and the frequency characteristics of the device are further improved; an appropriate choice of the thickness of the Si cap layer can effectively reduce the accumulation of carriers at an interface and increase the gain of the device; at the same time, the preparation method of the bipolar transistor is completely compatible with a 90-nanometer CMOS process, which effectively reduces the development and manufacturing cost of the device.