This paper reports strategies of sensitivity enhancement for magneto-optic surface plasmon resonance (MOSPR) sensors using metal heterostructure for two types of measured signals, ordinary and normalized reflectivity changes ( R PP and R PP /R PP ). The sensitivities using the R PP and R PP /R PP signals are effectively enhanced with the improvement as large as 58.91% and several orders by adjusting the device parameters. This paper demonstrates the occurrence of the maximal sensitivities for the R PP and R PP /R PP signals at different incident angles and composite film parameters for the first time. The sensing using the R PP /R PP signal requires a smaller total film thickness, a larger thickness ratio of the Co/Au film and a smaller thickness ratio of the underlay/overlay Au film than that using the R PP signal. Besides, the sensing using the R PP /R PP signal has a much higher sensitivity than that using the R PP signal. The conditions for maximal sensitivity and the physics of sensitivity enhancement for MOSPR sensors are also discussed. The newly-discovered ultra-high sensitivity using the R PP /R PP signal, which is several orders larger than that in the literature, has strict parameter tolerance and its realization requires accurate tuning of the device parameters.