The invention discloses a process for preparing synthesis ammonia from a synthesis gas and co-producing ethylene glycol. The process is characterized by comprising the following steps: carrying out desulfuration and decarburization on the synthesis gas after transforming and adjusting the ratio of CO to H2, then preparing a part of the transformed and purified gas after desulfuration and decarburization into high-purity hydrogen through an H2 purification process, conveying the high-purity hydrogen obtained by purification into an ethylene glycol synthesis device and preparing ethylene glycol through a hydrogenation reaction with dimethyl oxalate, conveying the other part of the transformed and purified gas after desulfuration and decarburization into a methanol synthesis device for synthesising methanol, separating out crude methanol synthesis gas through cooling and gas-liquid separation operation and conveying the crude methanol synthesis gas to a deep decarburization step to remove CO and CO2 therein, then adjusting the ratio of N2 to H2 in the system through nitrogen supplement and then conveying to an ammonia synthesis device to produce synthesis ammonia. The process is capable of realizing co-production of synthesis ammonia and ethylene glycol, and especially suitable for transformation and optimization for synthesis ammonia factories.