Recently, polymer gel used as fluid loss pill for well workover and completion is reported more, while it mainly focuses on using single crosslinking mechanism. To the best of our knowledge, polymer gel based on multi-crosslinking is commonly used in the aspect of water shutoff. The theory of multi-crosslinking for preparing polymer gel has rarely been used in overbalanced well workover and completion. In this paper, we conduct experimental study to reveal the physical process and fluid loss control mechanism of the polymer gel pill based on multi-crosslinking during overbalanced well workover and completion. We propose the design philosophy of the multi-crosslinking based on two or three crosslinking reactions under different conditions. In this paper, the first gelation reaction of the target polymer gel is between a kind of high-temperature resistance acrylamide based polymer (abbreviated as "SPAM") and fresh chromium acetate as metallic-ion crosslinker to quickly form a pre-crosslinking gel with strength of around Code C at surface temperature of 30 degrees C. It aims at guaranteeing pumping performance and restricting free water for lowering the risk of fluid loss in inner tube wall and or the separation of polymer and crosslinker during pumping process. Subsequently, with the increasing of wellbore temperature during the pumping process, the pre-crosslinking gel will gradually become mature (when gel reaches at the final gel strength) to form high strength gel by the use of either polyethyleneimine (PEI) or hexamethylenetetramine (HMTA) or both as organic crosslinker(s) through covalent bonding. The mature polymer gel can realize the complete crosslinking structure to guarantee gel thermal stability. The mature polymer gel pill based on multi-crosslinking formulated with a combination of SPAM, fresh Chromium acetate, PEI and HMTA indicates an obvious viscoelasticity characteristic at high temperature of 150 degrees C, while exhibiting a notable elastic performance for the formula without crosslinker HMTA. The fluid loss evaluation experiment result shows that the mature polymer gel based on multi-crosslinking can really act as fluid loss pill to withstand high positive pressure during overbalanced well workover and completion. Whilst there is some degree of permeability damage for the treated core, but not too high. Besides, the scanning electron microscope (SEM) is further employed to reveal the fluid loss control mechanism of the mature polymer gel at the micro level. This study provides an avenue to implement polymer gel pill based on multi-crosslinking for fluid loss control during overbalanced well workover and completion.