The invention belongs to the technical field of oil and gas field plumb shaft separate layer treatment and provides a partial pressure annular pressure control method of plumb shaft coiled tubing sand filling. The partial pressure annular pressure control method includes the following steps: (1) assembling a downhole tool string; (2) lowering in the downhole tool string, installing a throttle valve at a sleeved pipeline outlet of a fracture well head, and making an ejector to align at the first layer position to be modified by aid of a positioner to complete first layer fracturing modification; (3) injecting the needed amount of sand in the annular space, settling for 1h, detecting the sand surface and adjusting the height of the sand surface; (4) moving the downhole tool string, aligning the ejector at the second layer modification position, opening the annular space, controlling annular pressure by aid of the throttle valve to conduct sandblasting and perforating; (5) after the sandblasting and perforating is finished, continuing to increase the displacement to pres open the stratum, and then closing a sleeve gate to complete second layer annular main fracturing treatment; (6) repeating steps (3) to (5) to complete modification of all other layers. The partial pressure annular pressure control method solves the problems of negative influence on the packing effect and the modification effect caused by too low annular pressure and incapability in annular pressure control during perforating.