The invention provides a non-contact detection method for roundness of a cylindrical roller. According to the non-contact detection method, a measurement environment is a dark room; when in measurement, the cylindrical roller is vertically placed in a fixture of a vertical machine tool for fixing; a projection plate is arranged on one side of the machine tool, a photosensitive element is arrangedinside the projection plate, and the photosensitive element is electrically connected with a computer; the cylindrical roller is irradiated by means of horizontal parallel light emitted from a surfacelight source on the other side of the machine tool, the parallel light passes through the cylindrical roller and is projected on the projection plate to form two boundaries of light and shade, and the photosensitive element converts an image of the projection plate into an electrical signal and transmits the electrical signal to the computer; the computer establishes a rectangular coordinate system on the generated image, and records abscissas of two measuring points on a specified ordinate on the projection boundary, and the measured difference between the two abscissas is the diameter; andthe cylindrical roller rotates for a cycle in a fixed-axis mode, and a roundness error of the cylindrical roller is calculated by adopting the least square circle method. The non-contact detection method can effectively avoid the measurement errors in a contact type measurement method and has higher measurement precision.