The invention relates to a production technology for preparing a polyferric chloride coagulant from a seamless steel pipe acid-washing waste liquid. According to the invention, seamless steel pipe acid-washing iron-containing waste hydrochloric acid liquid is used as a raw material, and a step-by-step-oxidation method is adopted. During the initial phase, air or oxygen is used for oxidation, and nitric acid is used as a catalyst; and during the later phase, a trichloroisocyanuric acid oxidizing agent is used for oxidation. The reaction is completely thorough, and conversion efficiency is high. As isopropanolamine is used as a stabilizing agent, product quality is stable, and performance is excellent. Iron content of a product prepared by the method reaches 11.6% and above, basicity reaches 16.8% and above, and stability is good. When the coagulant is used in flocculation tests of simulation wastewater and urban domestic wastewater, turbidity removal rate reaches 90.0% and above. The coagulant has a good water purification effect. According to the technology, a polyferric chloride coagulant is prepared at normal temperature and at normal pressure without external heating, and energy consumption is little. The technology is simple. By the technology, the environmental protection purposes of dealing with waste by waste and turning waste into wealth can be achieved, cyclic utilization of waste is realized, and good economic benefit and social benefit can be generated.