This paper investigates the intensity tuning characteristics of a double longitudinal modes He-Ne laser subjected to optical feedback. The intensity undulations of the total light and the two modes are observed for different exteal cavity length. Two modulations of the inteal cavity length are performed. One is only for the inteal cavity length being modulated and the other is for both the inteal and the exteal cavity length being modulated. The undulation frequency of the total light is found to be determined by the ratio of exteal cavity length to inteal cavity length in both modulations. When the exteal cavity length is integral times of the inteal cavity length, the fringe frequency of the total light could be seven or even more times of that in conventional optical feedback. A simple theoretical analysis is presented, which is in good agreement with the experimental results. The potential use of the experimental results is also discussed.