The invention relates to a simulation transformer gas relay device and a sample gas preparation and acquisition method thereof. According to the method, through mutual cooperation of a main oil tank, an adjustment oil tank, a secondary oil tank and a transformer gas relay device, large-capacity standard transformer oil samples with random characteristic gas concentrations can be accurately prepared, online verification on oil chromatography in matching with a power transformer is convenient, components and contents of various types of characteristic gases in the oil are accurately controlled, and long-time constant-value preservation under the normal temperature can be realized. The prepared transformer oil can be outputted in a continuous and constant-pressure mode, use cost is low, operation is safe, and the method is rapid and convenient, the transformer oil employs a purifying and cycle use mode, and economical performance and environmental protection are realized.