We demonstrated a femtosecond Ti:sapphire oscillator at 10?MHz repetition rate by introducing a specially designed multi-pass telescope to increase the cavity length. Stable mode locking laser with average power of 200 mW is obtained under 532?nm laser pump of 5?W power, corresponding to a single pulse energy of 20?nJ. Based on this laser, we further explored the characteristics of pulse duration and spectrum with the net intracavity group-delay dispersion （GDD）. The result indicates that the near-bandwidth-limited pulse can be obtained when a little net negative GDD exists, the optimized pulse duration as short as 56?fs was generated in this case. The positive GDD will lead to a widened pulse with the increase of positive GDD, and the pulse duration can extend to more than 600?fs with the largest allowable positive GDD.