Abstract: A series of tests were conducted in a small scale tunnel to study the n-heptane pool fire oscillation characteristics. The flame oscillation frequency was extracted through the Matlab image processing tool. The relations between the dimensionless oscillation frequency St and Fr were analyzed under the varying tunnel width in this study. Results showed that the burning process of n-heptane pool fire can be divided into the initial burning stage, transition stage and the stable burning stage in the tunnel. In the initial and stable burning stages, the flame oscillation frequency was constant. The transition stage can be further divided into the intermittent ceiling jet flame stage and the continuous ceiling jet flame stage, and the dimensionless oscillation frequency St was proportional to Fr^-0.5. The tunnel width also had an influence on the n-heptane pool fire oscillation characteristics. In the intermittent ceiling jet flame stage, reducing the tunnel width will increase the mass burning rate, and the proportion coefficient will increase. In the continuous ceiling jet flame stage, reducing the tunnel width will lead to a decrease in the proportion coefficient due to the anti-buoyancy effect of the wall jet.