A symmetric side cavity coupling sensing structure based on two-dimensional photonic crystals is proposed for voltage and magnetic field strength sensing. In the complete photonic crystal, defects are introduced through the translation and size change of air holes, and two photonic crystal microcavities, H0 cavity and improved H1 cavity, are formed respectively; The H0 cavity and the improved H1 cavity are coupled with the W1 waveguide respectively, and the symmetrical structure is made along the W1 waveguide; The microcavity is filled with liquid crystal and magnetic fluid as sensitive materials, and the electro-optical effect of liquid crystal and the magneto-optical effect of magnetic fluid are used to form the sensing region of voltage and magnetic field intensity. Due to the photon localization characteristics of photonic crystals, two relatively independent transmission peaks are formed in the transmission spectrum of the side cavity coupling structure. The changes of voltage and magnetic field intensity are indirectly measured by measuring the wavelength offset of the two transmission peaks. The sensing characteristics are numerically studied under anisotropic perfectly matched layer boundary conditions using the finite-difference time-domain method. The simulation results reveal that the voltage sensitivity can reach 0.65 nm/V in the voltage range of 14-32 V and 1.86 nm/V in the voltage range of 32-50 V. In addition, the sensor demonstrates a refractive index sensitivity and quality factor of 296 nm/RIU and 3350 in the voltage range of 14-50 V and 251 nm/RIU and 2722 in the magnetic field strength range of 10-40 mT, respectively. And the magnetic field strength sensitivity is 13.06 nm/mT in the magnetic field strength range of 10-40 mT.