The present invention discloses a multi-beam type single-mass block in-plane biaxial acceleration sensor chip and a preparation method thereof. The sensor chip adopts an SOI silicon slice to manufacture, and comprises a chip outer frame, a master supporting beam, a connecting beam, a slave supporting beam, sensitive piezoresistive micro-beams, a mass block, metal lead wires and pads. One end of the master supporting beam is fixed to the chip outer frame, and the other end is connected with the connecting beam. The other end of the connecting beam is connected with the slave supporting beam, and the other end of the slave supporting beam is connected with the mass block. The eight sensitive piezoresistive micro-beams in the chip are located in the gaps between the chip outer frame and the connecting beam, every two sensitive piezoresistive micro-beams are distributed at the two sides of the master supporting beam symmetrically, one ends of the sensitive piezoresistive micro-beams are fixed to the chip outer frame, and the other ends are connected with the connecting beam. The piezoresistors on the eight sensitive piezoresistive micro-beams are connected via the metal lead wires andthe sixteen pads and form a Wheastone full-bridge circuit. The sensor chip of the present invention can realize the measurement of the acceleration less than 100g, has an inherent frequency more than40 kHz, and satisfies the high-frequency low-g value acceleration dynamic measurement requirement.