In China, most ambulatory care is provided by public hospitals. However, many public hospitals’ outpatient services fall short of meeting patients’ demands. This study aimed to assess the quality gap of outpatient service in public hospitals by using an indicator system developed based on the SERVQUAL model. This cross-sectional study was conducted from June to July 2019 at 13 public hospitals in Shenzhen. In total, 1876 outpatients were included in the study, and participants were asked to fill out the modified SERVQUAL questionnaire. The scale consists of 23 items scattered in 6 dimensions, including Safety, Reliability, Responsiveness, Tangibles, Assurance, and Empathy. Descriptive analysis, t-test or F-test, and optimal scale analysis were performed. The patients’ expectation of outpatient services exceeded the hospitals’ services they perceived and resulted in negative gaps between the mean expectation and perception scores for each indicator. The gaps were as follows: Reliability = Empathy > Responsiveness > Safety > Assurance > Tangibles. Quality gaps differed significantly between age, education, and hospital type. The overall impression mean scores are 7.45 ± 1.18 (out of 9 points) and 7.27 ± 1.23 for general and specialized hospitals. And the willingness to recommend the hospital to others mean scores are 4.06 ± 0.62 (out of 5 points) and 3.92 ± 0.65 for general and specialized hospitals. Patients’ impression of the hospital and willingness to recommend the hospital were significantly correlated with age, citizenship, health insurance, referral type, frequency of visits, Safety, Tangibles, Reliability, and Assurance. The outpatient service provided by Shenzhen’s public hospitals did not meet patients’ needs. Hospital administrators should evaluate the quality gap of outpatient services to identify the flaws within the delivery of ambulatory care and make improvements according to the findings to drive patient-centered care.