A method for spectrum cleaning and stability improvement of femtosecond optical parametric oscillator (OPO) based on intracavity dispersion management was proposed and demonstrated for the first time. For high-power femtosecond OPO, the output pulses generally have irregular broadband spectrum varying with time, and the output power show a remarkable fluctuation. Through employing lithium niobate (LiNbO₃) crystal to introduce additional negative dispersion in the cavity, transform-limited femtosecond pulses with cleaning and smooth spectrum by temporal filtering effect of pump pulses were realized, and the spectrum stability and power stability of femtosecond OPO were greatly improved. This method is a simple and flexible way to realize the spectrum cleaning and stability improvement of femtosecond OPO, and is of significant value for the development of high power ultrashort-pulse OPO.