Objective: To clarify the definition of pseudolock phenomenon during locking plate osteosynthesis, propose a novel clinically oriented classification, and explore the incidence and distribution characteristics of this phenomenon. Methods: According to the insertion state of locking head screws in the threaded holes of locking plates, the pseudolock phenomenon was classified into three types: 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 of 1 796 patients with upper limb long bone fractures (clavicle, humerus, ulna, and radius) who underwent locking plate osteosynthesis from June 2017 to April 2024 were collected and analyzed. 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. Results: A total of 1 878 locking plates were identified in the long bones of the upper limbs, distributed as follows: clavicle 1 129(60.1%), humerus 364(19.4%), ulna 137(7.3%), and radius 248(13.2%). In total, 11 636 locking head screws placed in these plates were included in the analysis, of which 2 564(22.0%)were pseudolocked. Type A pseudolock was the most common with an incidence of 14.1%(1 646/11 636), followed by Type B at 7.8%(911/11 636), while Type C was rare with an incidence of 0.1%(7/11 636). Conclusion: The pseudolock phenomenon in locking plate osteosynthesis is defined and a new classification system is established. The incidence of pseudolock exceeds 20% in osteosynthesis of upper extremity long bones including clavicle, humerus, ulna and radius. Mastering the preset insertion depth and orientation of screws for locking plates and identifying pseudolock timely via tangential fluoroscopy during operation are crucial measures to improve the fixation stability of locking plates.