By using nonequilibrium Green's functions in combination with the density functional theory, we investigate the spin transport properties of manganese porphyrin-based spintronic devices constructed by two manganese porphyrin molecules connected with a p-phenylene-ethynylene group. The interesting spin filtering and magnetoresistance effects can be observed in the device. Particularly, when the overlap of π channels between manganese porphyrin and phenyl ring parts is broken, the spin filtering efficiency and magnetoresistance ratio of the device can be effectively increased. Meanwhile, the transition from a giant magnetoresistance molecular device to a tunnelling magnetoresistance molecular device can also be realized by this simple operation process. Moreover, electrically induced switching behavior based on negative differential resistance effects is also observed in our model. The mechanisms are proposed for these phenomena.