The invention relates to a slag gasification reaction furnace for preparing carbon monoxide, and a gas preparation method thereof, belongs to the fields of coal chemical industry, coke (containing semi-coke) gasification, fuel gas and the like, and relates to chemical engineering, a chemical process, chemical machinery, an instrument and an automation technology. The slag gasification reaction furnace comprises a gasification furnace body, a slag hole, a combustion short section, a chilling chamber and an automatic coke adding machine, wherein the gasification furnace body comprises a shell, an upper barrel, a hearth inlet, a coal gas outlet, lining refractory bricks, a water jacket, a gasifying agent nozzle, a water cooling coil pipe, a slag pool, a furnace cover and a large connecting flange. According to the invention, due to the adoption of slag gasification and liquid deslagging, the gasification temperature is high, the carbon conversion rate is improved, the effective gas content is high, the operation energy consumption is low, the cost is low, the methane content is low, the raw coal application is wide, the investment is saved, the equipment is completely domesticated, the maintenance is convenient, the operation is simple and feasible, the requirements of safety, environmental protection and energy conservation are met, and the pure oxygen continuous gasification technology is upgraded.