Abstract: As a reducing agent for selective catalytic reduction (SCR) system of diesel vehicles, the quality of vehicle urea solution is crucial for ensuring compliance with NO_x emission standards. To evaluate the quality of vehicle urea solutions in typical cities, 45 samples from the sales-side and 50 samples from usage-side were collected and analyzed. 19 parameters, including urea content, density, refractive index, insoluble matter and biuret content, etc., were analyzed by using standard methods. A quality evaluation method for vehicle urea solutions was established to evaluate the sample quality, and relevant regulatory recommendations are proposed. The results indicate the following: (1) Non-compliance was observed in 15% of the urea samples, with 2% of the sales-side and 26% of the usage-side samples failing to meet standards. Greater attention should be paid to usage-side quality, as some vehicle owners may purchase substandard products from uncertified vendors or at low prices. Additionally, impurities introduced during refilling may also lead to these failures. (2) The primary non-compliant indicators were biuret content, insoluble matter, and urea content, with 8, 6, and 4 non-compliance samples, respectively. These indicators will affect the NO_x conversion efficiency of SCR system directly or indirectly. (3) A quality evaluation method was established, considering both non-compliance and the degree to which compliant samples approached optimal values. Weighted scores for urea content, density, refractive index, biuret content, insoluble matter, calcium, and sodium were 83, 91, 84, 66, 84, 97 and 97, respectively. Density, calcium, and sodium wereclassified as excellent, while urea content, refractive index, and insoluble matter were rated as good. Biuret content was rated as acceptable, althoughits overall quality was comparatively lower. (4) For future regulatory efforts, it is recommended to ensure that production and sales quality meets established standards through increased random inspection and stricter enforcement against non-compliant behaviors. Additionally, vehicle owners and enterprises should prioritize purchasing from certified suppliers and verifying product certification documents. Regulatory agencies may also consider tracking non-complianceat the usage-side back to production and sales sources for further investigation.