A photo-electronic window and windshield fog-rain detecting system comprising two sub-systems from a film-attached fog detecting assembly, a regular detecting and a rain detecting assembly, and a corresponding Gate-relation circuits to couple the two assemblies with each other electronically to make multi function detecting. The regular detecting assembly uses an emitter to transmit light signals towards a test area on the windshield and uses a sensor to receive signals portion from the test area to sense the changes in the received signal portion, caused by fog or rain, so as to verify the existence of for or rain on the windshield. The film-attached assembly is formed by applying an appropriate opaque thin film material to the test area, such as foil, plastic, paint, etc., to avoid meeting any influence by the outside fog and rain, so as to be able to reliably detect fog on the interior surface of windshield and handle the defogger. The rain detecting assembly is formed by using a covering mounting structure to completely cover the detecting devices on the interior surface of windshield to avoid meeting any inside fog, so as to be able to reliably detect outside fog or rain on the exterior surface and handle the wipers. By coupling any two of three assemblies together electronically, a reliably multi function fog and rain detector is built up to instantly actuate a right tool, wipers or the vehicle defogger, to clear the windshield up when fog or rain is on.