A MEMS microphone chip with an expanded back chamber includes a first chip unit and a second chip unit. The first chip unit has a first substrate, a vibration membrane layer is formed above an end of the first substrate, and a space is formed below the vibration membrane layer of the first substrate, so that the vibration membrane layer is suspended above the first substrate to vibrate. The second chip unit has a second substrate to couple with another end of the first substrate, and a groove is formed in the second substrate with a width larger than that of the space; when the first substrate and the second substrate are coupled together, the groove and the space are connected together to act as the back chamber of the vibration membrane layer.