A projection device and a projection image correction method are provided. Four target coordinates of four target vertices forming a target quadrilateral boundary are obtained. A first trapezoidal boundary is obtained according to a predetermined image boundary and a first coordinate component of each of the four target coordinates. At least one edge of the target quadrilateral boundary is extended until intersecting with at least one of two reference line segments to obtain a second trapezoidal boundary. Bases of the first trapezoidal boundary are perpendicular to bases of the second trapezoidal boundary. First direction scaling processing is performed according to the first trapezoidal boundary, and second direction scaling processing is performed according to the second trapezoidal boundary, to scale an original image into a target image block aligned with the target quadrilateral boundary in an output image. The projection device projects the output image to display a rectangular projection image.