An illumination system includes LED arrays (1, 2, 3) which are packaged on a heat dissipation plate and consist of several LED chips, an optical combining device (4) which can combine the light emitted by respective LED chips to one beam, a focusing lens (5) which can focus the light emitted by the optical combination device to a light-emitting window (11), a transmission type wavelength converter (7) and a control processing device. When white light is provided, the control processing device controls the LED chips with predetermined wavelengths to emit light and the transmission type wavelength converter is positioned into the light path to generate excited light.