A transmitter for online monitoring of moisture, hydrogen content, and temperature of a transformer, comprising an electrical box (1), a long detector tube (3), an installation adaptor (11), a sleeve fastening nut (9), an upper heat-insulating clip (19), a lower heat-insulating clip (20), a heat-insulating sealing step (21), an end cap (15), a flexible circuit conduction band (16), and the like. The long detector tube (3) extends into the interior of the electrical box (1) and is tightened and sealed by screwing a flange nut (2); the installation adaptor (11) and the sleeve fastening nut (9) are fitted over the long detector tube (3); an assembly for suspending and securing a flexible circuit board (24) is provided inside a detector end of the long detector tube (3); the assembly is assembled into a heat-insulating cylindrical body by means of the upper heat-insulating clip (19), the lower heat-insulating clip (20), and the heat-insulating sealing step (21); the end cap (15) is threaded onto the long detector
