The invention discloses a main steam valve of a steam turbine. The main steam valve comprises a valve body, a bonnet and a valve stem, wherein an upstream steam passage, a downstream steam passage, a valve stem assembly hole, a bonnet assembly hole and a valve seat are formed on the valve body. The bonnet mainly comprises an inner bonnet, an elastic sealing ring, a hold-down ring and a locating ring which are arranged in the bonnet assembly hole from the bottom up in sequence, and an outer bonnet which is fixed at the outer end of the bonnet assembly hole through fixing bolts, wherein the inner bonnet is subjected to self-tightening seal and bears most pressure, the outer bonnet bears less pressure, and the sealing effect is good. The upper end of the valve stem passes through a sleeve and is connected with a valve disc, and under the operation of the valve stem, the valve disc is matched with the valve seat to close or open the upstream steam passage and the downstream steam passage on the valve body. A sealing cartridge is arranged on the sleeve, and a sealing cone is arranged on the valve stem. When the valve disc and the valve seat are open, the valve stem is sealed with the sealing cartridge through the sealing cone, and the opening of the valve disc is limited by the sealing cone. The main steam valve has the characteristics that the main steam valve is good in sealing performance, low in production cost, high in practicability and capable of bearing high parameter steam.