The invention provides an airtight seal thread structure of a magnesium alloy sacrificial anode for a water heater. The airtight seal thread structure is arranged at a joint of a magnesium alloy anode bar and a nut, anode threads are arranged at an anode end of the magnesium alloy anode bar and are divided into a connecting thread section and a protruded thread section, and the connecting thread section and the protruded thread section are of NPT (national pipe thread) structures. The airtight seal thread structure of the magnesium alloy sacrificial anode has the advantages that leakage channels which are formed among tooth crests and tooth bottoms after male threads and female threads of the anode and the nut are fittingly screwed are plugged, so that an airtight seal effect can be realized; the sealing performance of an anode product can be guaranteed by the airtight seal thread structure as compared with a hydraulic seal effect realized by a thread structure and welding at present, the problem of leakage of welding spots during production can be completely solved, and the airtight seal thread structure is favorable for improving the stability and the safety of the product; the airtight seal thread structure is optimized and accordingly is applicable to all GB/T17731 round bar magnesium alloy anode products with various specifications and other non-standard round bar anode products customized for clients.