An extrinsic Fabry-Perot interferometer (EFPI) consisted of coupling quartz diaphragm and fiber optic pigtails is designed to detect acoustic signal generated by partial discharge (PD) in liquid-solid dielectric insulation.To solve the problem of low sensitivity,based on the theory of elasticity the structure parameters of EFPI diaphragm are determined using finite element analysis method,and EFPI samples are manufactured.In order to demodulate acoustic signal,a quadrature intensity demodulation system is established using distribute feedback (DFB) laser.A piezoelectric transducer (PZT) and EFPI sensor contrast measurement system is designed using needle-plate electrode to produce partial discharge.Results show that the sensitivity of EFPI sensor is determined by response frequency and static pressure sensitivity of EFPI.Design method of EFPI diaphragm is improved,and an EFPI sensor which has the same sensitivity as PZT for partial discharge detection is designed.