The present invention discloses a synthetic ammonia system for making hydrogen by electrolysis in a thermal power plant includes an electrolytic hydrogen making device and a synthetic ammonia equipment; a power input end of the electrolytic hydrogen making device is electrically connected with a power generation output end of the thermal power plant; a hydrogen output end of the electrolysis hydrogen making device is connected with a hydrogen inlet of the synthetic ammonia equipment, a nitrogen inlet of the synthetic ammonia equipment is connected with a nitrogen source, the synthetic ammonia equipment is used for using the hydrogen produced by the electrolysis hydrogen making device and nitrogen of the nitrogen source to synthesize ammonia; an ammonia output end of the synthetic ammonia equipment is communicated to an ammonia supply pipeline and/or a liquid ammonia tank of the thermal power plant.