When operating at high elevation of 3700m (atmospheric pressure about 68 kPa), the combustion process of diesel engine deteriorates, and the engine performance declines significantly. In this paper, Isooctyl Nitrate(EHN) is blended into the diesel fuel as additive to improve the combustion process. The decomposition of Isooctyl Nitrate(EHN) is analyzed and its mechanism is studied through chemical kinetics. A series of tests were carried out on a single cylinder diesel engine to study the effects of EHN on diesel engine combustion with the low intake pressure of 68kPa. Results show that the generation of OH、 H、 HO2 and H2O2 in n-heptane cleavage reactions can be promoted by EHN. In both stages of low and high temperature, the decomposition of n-heptane is accelerated, which shortened the ignition delay period. Four kinds of fuel are studied by tests: diesel fuel, diesel fuel with 0.3%, 0.6% and 0.9% mass fraction EHN respectively. Compared with diesel fuel, the peak cylinder pressure decreases 0.06 MPa, 0.09 MPa, 0.13 MPa with different amounts of EHN under low intake pressure condition. The peak value of Instantaneous Heat Release Rate decreases 5.0%, 8.0%, 9.8%, and the curve of Instantaneous Heat Release Rate moves forward 0.4 °CA, 0.8 °CA, 1.1 °CA. In addition, engine output torque increases, while Brake Specific Fuel Consumption decreases. The dynamic property and fuel economy of diesel engine are improved.