The invention relates to a three-post plastic-pipe pressure-resistance combined oil tank, which is characterized in that: a fuel oil tank body is arranged on one side of a combined oil tank, urea reducer pressure-resistance pipe bodies are arranged in the middle, and an electrical control box is arranged on the other side; a seaming end cap batten is arranged on one side of the electrical control box; each urea reducer pressure-resistance pipe body is provided with a urea reducer box cover and is communicated with a compressed air power input pipe and a urea reducer output pipe; a liquid level sensor is fixed on each urea reducer pressure-resistance pipe body, a urea reducer tap valve is arranged at bottom of each urea reducer pressure-resistance pipe body, upper and lower pipe plugs are provided with elastic supporting glue piles, and plastic pipe clips are fixedly connected with the box body; each urea reducer pressure-resistance pipe body is fused with the upper and lower pipe plugs by a fusion pair pipe to form a post, and three urea reducer pressure-resistance pipe bodies having the same structure are connected into a whole by two groups of fusion region connectors; and after the fusion forming, a 30-minute hammering seal test is performed under a constant pressure of 0.3 to 0.7Mpa. With compact design, the oil tank helps to realize the elegant design of a vehicle, reduces connectors, simplifies vehicle configuration, reduces vehicle weight, and can meet national above 4 standard class rate (SCR) standard requirements and reduce system cost.