As a typical partially premixed flame, a Triple Flame widely exists in internal combustion engines. In this study, experiments were carried out on a propane triple flame. The base propagation characteristics of the attaching flame and the lifted-off flame of a propane triple flame under the condition of coflow were investigated. The base height, base width, and geometric structure of the triple flame were analyzed. The results showed that a higher coflow equivalence ratio is more beneficial in shortening the flame attaching time, and the increase in mainstream velocity interferes with the combustion of the mixture of the upstream flow field. In addition, the entrainment mixing of the fuel and ambient air increases the downstream combustible range, thus increasing the flame base width. For lifted-off flame, an increase in the mainstream equivalence ratio decreases the local radial combustible range of the flame base front position, thus decreasing the base width. A lower mainstream velocity decreases the flame propagation resistance, which is most beneficial for flame stability in the flow field, and decreases the mainstream velocity of the flame that appears to be in the blowing-out state. At a low mainstream equivalence ratio and low mainstream velocity, the amplitude of flame oscillation is relatively weak, and the mainstream velocity is the main influencing factor for this phenomenon.