A projection system, a projection image adjusting method and a projector are provided. The projection system includes a projector and a control unit. The projector projects a projection image. The control unit controls movement of an image grid point of the projection image projected by the projector. When the projector receives a continuous adjustment signal output by the control unit, the projector determines a cumulative displacement amount proportional to a continuous signal quantity cumulative value according to the continuous signal quantity cumulative value of the continuous adjustment signal, and the image grid point is moved by the projector according to the cumulative displacement amount to correspondingly deform at least a part of the projection image, so as to provide a convenient projection image adjustment effect.