The invention provides an underground autogenous mixed-phase thermal fluid swallowing-spitting method and relates to the technical field of oil production in oil fields. The method comprises the following steps: carrying out well-completion practice on a target swallowing-spitting well, then arranging an underground mixed phase thermal fluid generator and a matched ground supply system underground after well completion, producing a mixed phase thermal fluid underground and injecting an oil layer, sealing the wall after injection rate reaches a preset range, and carrying out oil production operation after well sealing time reaches a preset range. The mixed phase thermal fluid is injected into the oil layer for carrying out swallowing-spitting oil production, carbon dioxide and high temperature steam in the mixed phase thermal fluid can modify crude oil, content of heavies, colloids and asphaltenes in the crude oil is reduced, the effects of reducing the viscosity of the crude oil and enhancing liquidity of the crude oil are achieved, and the crude oil can be produced more easily; and the method provided by the invention can be applied to primary oil production of heavy oil and super heavy oil reservoirs difficult to be put to use and also can be used for tertiary oil production of the heavy oil and super heavy oil reservoirs.