An optical system of a semi-vertical single LCD (liquid crystal display) projector includes a projection light source, a condenser, an illuminating reflector, a collimating lens, a heat reflecting glass, an LCD light valve, a field lens, an imaging reflector and a projection lens, all of which are set in sequence according to a direction of light, wherein the illuminating reflector reflects and turns light irradiating the LCD light valve in up and down directions, and the imaging reflector performs mirror reflection and turning on light emitted by the LCD light valve in left and right directions. The present invention obtains a new optical system structure, creates a novel stacking and a novel projector appearance, and has low cost, small size and complete integration.