A flat-top sinusoidal phase mask coronagraph is theoretically studied, it is the generalized form of the sinusoidal phase mask coronagraph and the six-level phase mask coronagraph. This kind of phase mask is a double-periodic phase mask with both the flat-top sinusoidal and the constant parts in the azimuthal phase in each period. The wideband working condition for this kind of phase mask is analytically presented. On one end, the wideband working condition reduces to that of a sinusoidal phase mask. And on the other end, it reduces to that of a six-level phase mask. Just like its two special examples, the numerical calculation shows the flat-top sinusoidal phase mask coronagraph generally has the advantage of achromatism.