Objective: Locking plate osteosynthesis is one of the most widely used methods for fracture fixation. These components, the locking head screw (LHS) and the locking plate (or locking brace), engage via matching threads to create an angularly stable construct. This provides reliable mechanical support and promotes fracture healing. However, the nonstandard placement of locking head screw remains a concern. The present study aims to introduce a new classification and describe the epidemiology of the pseudolock phenomenon in upper limb locking plate osteosynthesis. Methods: Herein, a new classification was proposed. Type A: the screw depth is insufficient, but the orientation is normal in a precontoured plate. Type B: both the screw depth and orientation are abnormal in a precontoured plate. Type C: the screw depth and orientation are either/both abnormal in an overbent plate. Demographic and radiographic data from patients with upper limb long bone fractures (clavicle, humerus, ulna, and radius) treated with locking plate osteosynthesis were retrospectively analyzed. The placement status of each locking head screw was assessed by three observers using the new classification system. A Chi-square test with Bonferroni correction was used to assess the incidence of pseudolock incidences across different genders, ages, sites, sides, locations, and diameters. The study cohort comprised 1796 patients who underwent locking plate osteosynthesis at our hospital between 01/06/2017 and 30/04/2024. Result: A total of 1878 locking plates were identified in the long bones of the upper limbs, distributed as follows: clavicle 1129 (60.1%), humerus 364 (19.4%), ulna 137 (7.3%), and radius 248 (13.2%). In total, 11636 locking head screws placed in these plates were included in the analysis, of which 9072 (78.0%) were standardly locked, and 2564 (22.0%) were pseudolocked. Among the pseudolocked screws, 1646 (14.1%) were classified as Type A, 911 (7.8%) as Type B, and 7 (0.1%) as Type C. Conclusion: This study first proposed the pseudolock concept. The new pseudolock concept and classification aid in characterizing the placement status of locking head screws and is a significant supplement to the theoretical system of internal fixation in orthopedics. The incidence of pseudolock phenomenon in locking plate osteosynthesis of the upper limb is more than 20% and the pseudolock phenomenon cannot be ignored in clinical practice. The standardized placement of LHS and the tangential fluoroscopy during surgery can effectively identify pseudolock phenomena. This helps to improve the reliability of locking plate osteosynthesis and better promote fracture healing.