The number concentration and size distribution of soot aerosols are imperative parameters in fire detection which is closely related to safety management, risk assessment, and fire analysis. The experiment of n-heptane pool fire is conducted in a pressure chamber which simulates the 101, 90.9, 80.8, 70.7, 60.6, and 50.5 kPa environments. The influence of air pressure on the number concentration and size distribution of soot aerosols is investigated. The DMS500 fast particulate analyzer is used to analyze the number concentration and size distribution of gas samples. Both the geometric mean diameter and count median diameter of smoke particles increases with pressure. A second-order polynomial equation with correlation coefficient of 0.99 is fitted to the geometric mean diameter of particles against pressure. The particle number concentration is significantly decreased when pressure is reduced according to a linear relationship with correlation coefficient of 0.92. The environmental pressure puts influence on the soot optical properties that affect fire smoke detection technologies used in the regions of different altitudes or air pressures.