A parameter adjusting method is applied to a projector having an ambient light sensor, a database and a digital micromirror device. The parameter adjusting method includes analyzing a detection signal generated by the ambient light sensor to acquire an environmental light datum, comparing the environmental light datum with a lookup table of the database to compute at least one compensation parameter, and adjusting an amount of reflection light generated by the digital micromirror device in accordance with the at least one compensation parameter for controlling the projector to output a calibrated projection image in response to compensation of the environmental light datum.