The double-layered perovskite La1.8Ca1.2Mn2O7 ceramic was synthesized by traditional solid-state reaction after heat treatment for two times at high temperature. Its structure, magnetic and electrical properties were characterized by x ray diffraction (XRD), scanning electron microscopy (SEM), HL5500PC and physical properties measurement system (PPMS). It has a double-layered Sr3Ti2O7 tetragonal structure with lattice constants a, b=3.901 (1 =0.1 nm) and c=19.369 , and unit cell volume V=295.21 3. The Curie temperature TC is 177 K. The I-V characteristic shows a nonlinearity increasing with decreasing temperature at low temperatures below the magnetic critical point of TC and the nonlinear coefficient α goes up to a maximum of 68.6 when the temperature goes down to the lowest of 14 K in this experiment. However, the nonlinearity disappears when temperature goes above the magnetic transition point of TC. It indicates that a magnetically correlated Schottky barrier between grain boundaries is responsible for the I-V nonlinearity when temperature is below the Curie temperature. A maximum value of 0.18 eV was obtained for the magnetic Schottky barrier at 14 K after using a back to back pn junction model to describe such a potential barrier between grain boundaries.