In this work, the pressure drop and flow characteristics of solid–gas two-phase flow in a horizontal pipe were investigated experimentally. The pressure drop was measured by pressure transmitters, and the particle flow behaviour was observed using a high-speed camera. The particle concentration and gas pressure drop were systematically compared under different supply standard air fluxes. It was found that the particle materials exhibited a dune flow at a lower airflow velocity, and the pressure drop decreased first and then increased with increasing supply airflow velocity, until the critical wind speed for spraying materials was achieved. The coefficient of variation and power spectral density (PSD) analysis of the pressure drop were introduced to study the flow stability in pneumatic conveying; the fluid flow stability increased with decreasing coefficient of variation, and the results of the PSD analysis were consistent with those of the flow characteristics and pressure drop. When the standard airflow velocity exceeded the critical wind speed, the solid–gas ratio had a slight impact on the flow stability, and the optimum supply pressure for pneumatic conveying of these spraying materials was 0.4 MPa.