Abstract: Idle is one of the important operating conditions for non-road diesel engines. In China III, IV stage emission regulations from diesel engines for non-road mobile machinery, the proportion of idle condition in the eight-condition steady-state test cycle is 15% for the diesel engine operating at non-constant speed, and its ratio is 5% in the six-condition steady-state test cycle for the diesel engine with rated net power less than 19 kW and operating at unsteady speed. In China IV stage of non-road transient cycle (NRTC), the idle condition accounts for 4.52%. Due to the fact that the idle condition does not output power and also the fuel injection amount is small, the air-to-fuel ratio is relatively large and the combustion is incomplete at this time, which results in a large amount of fuel consumption and high emission of harmful gases. The land area with an altitude of more than 1 000 m in China is about 58% of the total area of the country, and proportion is about 33% for the plateau area with an altitude of more than 2 000 m. A large number of agricultural machinery powered by diesel engines operate at these areas. Compared with the sea level, atmospheric pressure will decline in plateau areas, the intake air mass flow rate decreases, and the operating performance at idle condition will further deteriorate. In order to investigate the effects of atmospheric pressure on combustion characteristics and emission performance of a small agricultural diesel engine at idle condition, the change law of combustion and emission performance of the small agricultural diesel engine, which meets the emission standard of China III non-road mobile machinery, was studied at different atmospheric pressures such as 80, 90, 100 kPa by using atmospheric pressure simulator. The experimental results demonstrated that the turbocharger matched with the small agricultural diesel engine did not work, and also the turbocharger system did not have self-compensation capability in plateau area at idle condition. Peak in-cylinder pressure, peak in-cylinder pressure rise rate, and peak heat release rate all go up with an increasing of atmospheric pressure; the average increase rate were 13.33%, 37.24% and 6.76%, respectively, with atmospheric pressure increased per 10 kPa. However, peak in-cylinder combustion temperature showed a decreased tendency with atmospheric pressure increased; the average decrease rate was 11.18% with atmospheric pressure increased per 10 kPa. Atmospheric pressure had a great influence on carbon monoxide (CO) emission and hydrocarbon (HC) emission. CO emission and HC emission declined sharply with an increasing of atmospheric pressure; the average decrease rate were 47.47% and 55.77%, respectively, with atmospheric pressure increased per 10 kPa. The variation range of nitrogen oxides (NOx) emission and smoke were relatively small with atmospheric pressure increased. With atmospheric pressure increased per 10 kPa, the average NOx emission increased 18.93%, while the average smoke reduced 11.90%. This research can provide a reference for the establishment of emission control strategy for a small agricultural diesel engine at idle condition in a plateau area.