Abstract: This study aims to investigate the impact of circulation injection quantity on the cold start and emission performance of small agricultural diesel engines. The optimal quantity of circulation injection was also determined at critical temperatures. The study focuses on an inline twin-bar high-pressure common rail diesel engine. Using the environment simulation cabin whole engine test platform, several pre-tests were conducted. The results showed that the diesel engine started successfully without opening the preheating device at an altitude of 2 000 m and a critical temperature of -5 ℃. Three types of injections were used in this case. Specifically, the injection time of pre-injection 2, pre-injection 1 and main injection was -22, -14.4 and -7.7 °CA ATDC (crank angle after top dead center), respectively, and the injection duration of pre-injection 2 was 2.5 °CA, and the circulation injection quantity was fixed at 2.5 mg. The pre-injection 1 injection duration was 3 °CA, and the pre-injection amount was fixed at 3 mg. The circulation injection quantity was varied in the main injection volume, with the values of 8, 15, 17, 20, 25, 30, 34, and 40 mg. The target speed was set as 1 050 r/min. The cold start time and emissions of HC, CO, and NOx were measured at various circulation injection quantities during the test. The results indicate that the cold start time of the diesel engine increased or even resulted in the start failure with the larger or smaller amount of circulation fuel injection. As the circulation injection quantity increased, the cold start time initially decreased and then increased again. The shortest start time was 266 s at the 20 mg of circulation injection quantity. The longest start time was 365 s at the 15 mg of circulation injection quantity, with a difference of nearly 100 s. Additionally, the lowest peaks of HC and CO emissions were also observed as 12 984 and 3 008 mg/L, respectively, at 20 mg. The variation in the circulation injection quantity also resulted in an increase in the peak of HC and CO emissions. Furthermore, the peak value of CO emission increased significantly with the larger circulation injection quantity, where the peak value appeared earlier. The circulation injection quantity had a greater effect on the NOx emission peak. The lowest peak of NOx emission was 368 mg/L at the circulation injection quantity of 30 mg. Therefore, there was a significant impact of circulation injection quantity on the cold start and emission performance of diesel engines. The optimal quantity of circulation injection was also beneficial to reducing the start time and emissions for higher generating efficiency and a friendly environment. Furthermore, 20 mg was the optimal circulation injection quantity for the cold start in this small diesel engine at an altitude of 2 000 m and an ambient temperature of -5 ℃. These findings can also provide experimental data to optimize the cold start and emissions of engines. The circulation injection quantity with after-treatment devices can further reduce the cold start emissions.