This study developed and validated an instrument to investigate senior school students' understanding of electrostatics and provide a cognitive diagnostic assessment of their strengths and weaknesses on the related concepts (e.g., electric charge). The instrument included 20 four-tier multiple-choice items and the development process is organized around two activities: the development of the instrument and its validation. The development step defined the secondary concepts and designed the items using the misconceptions related to them. In the validation step, the instrument was applied to 1850 senior high school students from nine schools in two provinces in China, and the collected data were analyzed using the CDM package in R language. This step ensures that the diagnostic reports represent students' conceptual understanding reliably and validly by selecting the best model, analyzing item quality, overall test reliability, and the instrument's structure. The instrument can provide the percentage of students in the test population who possess certain combinations of concepts, the percentage of students in the test population possessing individual concepts, and the fine-grained size of concept proficiency information, which can be integrated as one completed report to issue to students, teachers, and parents to demonstrate students' status of conceptual understanding related to electrostatics. In addition, the construct induced from the diagnostic results can also be aggregated to the classroom, schools for instruction planning or low-stakes decision making, or infer a learning sequence.