The invention provides an installation device used for a vehicle headlamp assembly and an installation method of the installation device used for the vehicle headlamp assembly. An installation base on the headlamp assembly is installed on a headlamp installation area of a fender-guard framework of a vehicle. The installation device comprises a plurality of adjustment positioning pins, the adjustment positioning pins can be inserted into a pin hole of the installation base and a positioning hole of the headlamp installation area, and have different eccentric distances between the central axes of pin handle parts and the central axes of pin head parts respectively, one of the adjustment positioning pins with the different eccentric distances is suitable for being selected to be inserted into the pin hole and the positioning hole to adjust the position of the installation base relative to the headlamp installation area so as to enable the outer surface of the headlamp assembly to be flush with the outer surface of a vehicle body, and the adjustment positioning pins further comprise rotary locking devices in locking fit with the pin hole or the positioning hole.