Based on constituent equations of longitudinally polarized piezoelectric materials and magnetized magnetostrictive materials, and equation of motion of the magnetoelectric element, the expression to calculate the open circuit voltage of the piezoelectric material is presented. In view of the characteristics of high impedance for the piezoelectric materials, and considering the capacitance of the lead cable to transfer signal, as well as the capacitance and the internal resistance of the measuring instrument, a formula to calculate the actual magnetoelectric voltage is derived by using an equivalent circuit. Taking different materials parameters, we calculate the effect of parameter variation on magnetoelectric voltage. The results show that the influence of material parameters, cable capacitance and the input capacitance and resistance of the measuring instrument on the output voltage must be taken into account. Non-uniformly biased field will yield forces on the two ends of the magnetostrictive materials and the research result shows that the constant force produced by gradient bias magnetic field will generate stabilized voltage, which is unable to be measured by the oscilloscope.