The invention discloses a MEMS fluid density sensor chip based on in-plane resonance and a preparation method thereof. The chip comprises a silicon substrate, four elastic fixed support beams and an intermediate vibrator. Two leads along the length direction of the silicon substrate are respectively arranged on the fixed support beams in two sides of the vibrator. A magnet at the bottom of the chip is used for providing a constant magnetic field. The direction of the magnetic induction line is perpendicular to the chip plane. Each one of the leads is applied with sinusoidal alternating currents having a certain frequency and the lead carries out in-plane vibration under action of an alternating Lorentz force on the fixed support beams so that the vibrator and the fixed support beams in the other side are driven to produce forced vibration and the other lead cuts the magnetic induction line and produces the induced electromotive force. The H-type silicon micro-double-end fixed support beam structure is immersed in the fluid to be detected and the frequency of the sinusoidal alternating current is changed so that fixed support beam resonance is produced. According to the amplitude of the output of the induced electromotive force, resonance frequency of the fixed support beams in the detected fluid is acquired. Through change of the fixed support beam resonant frequencies in different fluids, fluid density can be measured.