The invention discloses a self-assembled film mercury ion selective electrode (1) which comprises a substrate (11) of a working gold film (12) modified by a 11-aminohendecylmercaptan film layer (25), wherein the working gold film (12) covered on the substrate (11) is connected with a pin gold film (13) through a gold film layer; the pin gold film (13) is connected with a metal wire (15) coated with a plastic insulation layer (14) through soldering tin (16); the substrate (11) is prepared by gradually spattering and depositing a silicon dioxide film layer (22), a metallic chromium film layer (23), a copper alloy film layer (24) and the gold film layer on a polished silicon borate base plate (21); moreover, the modified 11-aminohendecylmercaptan film layer (25) is assembled on the surface of the working gold film (12). The selective electrode is simple to manufacture, low in cost and convenient to use, and can be used for quickly detecting trace amounts of mercury ions in the environment; the lower limit of detection is 6.3 nmol/L.