The invention relates to the field of screen pipe completed horizontal wells of oil/gas wells, in particular to a slurry displacement process for a screen pipe completed oil and gas well. The process comprises the following steps of: displacing with a certain volume of reservoir protection liquid at the bottom of the well after a wellbore is drilled and before a screen pipe is descended; descending the screen pipe, wherein the reservoir protection liquid which is displaced for the first time is completely or partially squeezed to fill annular spaces of the wellbore and the screen pipe during the descending of the screen pipe; and displacing slurry in the screen pipe with the reservoir protection liquid by descending a drilling tool to the bottom of the well after the screen pipe is descended. By the process, the slurry inside and outside the screen pipe of a hydrocarbon reservoir section of the screen pipe completed well is completely displaced, a hydrocarbon reservoir is protected well, and the well production rate and the hydrocarbon reservoir recovery rate are improved. The defect that the slurry outside the screen pipe cannot be displaced by the conventional process is overcome.