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In the framework of Einstein-Maxwell-scalar theory, we studied scalar field perturbations of Reissner-Nordström-Melvin (RNM) black holes, which describes the RN black holes immersed in a uniform magnetic field. Due to the coupling to the Maxwell term, the scalar field acquires an effective mass square which, in the presence of the magnetic field, will become negative somewhere outside the horizon for either sign of the coupling constant α, thus triggering the tachyonic instability and leading to spontaneous scalarization when α is large enough. The magnetic field has significant influences on the waveforms and the onset of the instability, which differs for different sign of α. Effects of the black hole charge on the instability are also studied.