A MEMS speaker includes a substrate, a vibration sounding portion and a baffle plate with a through hole. The baffle plate, the substrate and the vibration sounding portion form a sounding inner cavity, and a volume of the sounding inner cavity can adjust a resonant frequency of the sounding inner cavity, so that the resonance frequency of the sounding inner cavity resonate with a preset frequency of the MEMS speaker. A speaker assembly structure further provided includes a speaker, a fixing portion, and a baffle plate, the speaker and the baffle plate together enclose and form a sounding inner cavity, the fixing portion and the speaker are fixedly connected together and form a sealing structure. A sound pressure level of the MEMS speaker and the speaker assembly structure is high and harmonic distortion of the MEMS speaker and the speaker assembly structure is small.