The invention relates to the technical field of underbalanced well completion and oil production, in particular to a method for underbalanced tubing descending, non-well-killing gas lifting, rotary pumping, pump maintaining and tubing maintaining. The method includes the steps: firstly, fixedly mounting an in-tubing blowout preventer and a lifting basket at the bottom end of a tubing string in sequence from top to bottom, and mounting a float valve in the in-tubing blowout preventer. The in-tubing blowout preventer is fixedly mounted at the bottom of the tubing string, so that tubing descending operation can be completed smoothly in the well completion phase, a gas lift liquid discharge or production passage can be established during gas-lift oil production, tubing descending, gas-lift oil production, oil pumping, oil-well pump maintenance and tubing maintenance can be closely integrated, operation cycle is shortened, operation of putting into production is simplified, reservoir protection effect of underbalanced well drilling is enhanced, oil-gas wells can be put into production timely to keep an excellent productivity increase effect, further competitiveness of the underbalanced technique is improved, and oil production cost is lowered.