The invention provides a lowering method of a steel protection casing, and relates to the hoisting technology field of the steel protection casing. The lowering method of the steel protection casing includes: step 1, supporting the steel protection casing on a steel platform, using a pressure bearing plate for blocking the top of the steel protection casing, blocking the bottom of the steel protection casing through an external medium so as to form a sealed inner cavity in the steel protection casing; step 2, inflating the inner cavity until air pressure Pi of the inner cavity reaches a presetvalue; step 3, relieving support for the steel protection casing; and step 4, controlling discharge speed of gas in the inner cavity so as to lower the steel protection casing. The lowering method ofthe steel protection casing forms the sealed inner cavity in the steel protection casing by sealing two ends of the steel protection casing, and then inflates the inner cavity until the air pressurePi reaches the preset value, and meanwhile the pressure bearing plate at the top of the steel protection casing is subject to upward pressure, and the pressure can counteract a part or all of the gravity of the steel protection casing, thereby improving hoisting capacity of a hoisting device.