The invention provides a methanol synthesis system and a steam dilatation method of the methanol synthesis system. The methanol synthesis system comprises a feed gas pretreatment device connected with a feed gas delivery pipe, a methanol synthesis reactor connected with the feed gas pretreatment device, and a boiler feed water pipe connected with the methanol synthesis reactor to enable boiler feed water to perform heat exchange in the methanol synthesis reactor so as to output byproduct steam, wherein a boiler feed water preheater is arranged on the boiler feed water pipe, and a heat supply inlet of the boiler feed water preheater is connected with a gaseous product output port of the methanol synthesis reactor so as to preheat the boil feed water in the boiler feed water preheater with a gaseous product in the methanol synthesis reactor and then output the gaseous product. By means of the technical scheme, the problem that heat output from the methanol synthesis reactor in the prior art cannot be effectively carried away or utilized and accordingly the temperature of waste heat of an output product is high can be solved.