Abstract: A method for online modeling illumination design is applied to communicate an illumination modeling system with a user end via an Internet so as to provide for a user to execute an online operation of the modeling illumination design. The method comprises the steps of: pre-writing an illumination modeling program; pre-storing at least one illumination device with at least one device parameter thereof; inputting at least one environment parameter to establish a modeling space; selecting and inputting at least one illumination device from the pre-stored illumination device; inputting an installation position for installing the illumination device; inputting an illumination position; and using the illumination modeling program, according to the environment parameter and the device parameter, to calculate a modeling illumination value of that the illumination device provides to the illumination position when the illumination device is installed to the installation position.