The invention discloses a low-cost and simple unit round-to-square plug nozzle test device, which comprises an igniter assembly, an injector assembly, a thrust chamber assembly, a plug cone assembly, a thrust measuring support table frame assembly and an ejection exhaust tube, wherein the injector assembly is provided with a hydrogen cavity and an oxygen cavity, a hydrogen and oxygen gas direct flow spray nozzle with the single hydrogen and oxygen injection function is arranged in the hydrogen cavity, the igniter assembly formed by three layers of sleeves is arranged inside the injector assembly, the injector assembly is connected with the thrust chamber assembly with a round-to-square inner nozzle, the plug cone assembly and the thrust chamber assembly are connected, are supported by the thrust measuring support table frame assembly and can forwards and backwards slide on the support table frame assembly, a platform with an installing thrust frame is also arranged on the support table frame assembly, and the back part of the plug cone is positioned in the ejection exhaust tube. The low-cost and simple unit round-to-square plug nozzle test device has the advantages that the structure is simple, the measurement precision is high, the cost is low, in addition, the application range is wide, the study content is rich, and the repeated use for many times can be realized.