The invention discloses a reactor used for preparing gas-phase hydrogen cyanide. The reactor used for preparing gas-phase hydrogen cyanide comprises an upper sealing head, a lower sealing head, a shell side and reaction tubes, wherein the reaction tubes are arranged in the shell side; the upper sealing head and the lower sealing head are respectively and correspondingly mounted at the upper end and the lower end of the shell side; the reactor used for preparing gas-phase hydrogen cyanide also comprises a feed gas distribution device, a temperature monitoring device, a heat-transfer medium flow guide device and a hollow flow guide cylinder; the feed gas distribution device is mounted in the top end of a main body structure of the tubular reactor; the temperature monitoring device is inserted from the top end of the main body structure of the tubular reactor; the heat-transfer medium flow guide device is mounted on the outer part of the shell side; flow guide holes are formed in an inner wall surface, being in contact with the heat-transfer medium flow guide device, of the shell side; the hollow flow guide cylinder and the reaction tubes are mounted in the shell side in a mixing manner. The reactor used for preparing gas-phase hydrogen cyanide has the beneficial effects of being capable of generating easily-separated hydrogen cyanide gas and stably and effectively dissipating reaction heat, an accurate reaction temperature monitoring system is arranged, the feed gas distribution effect is good, the reaction conversion rate is high; a catalyst is low in loss during use and long in service life.