A new method of measuring molecular line parameters based on the transmitted fringes of the F-P cavity was reported. A transmitted function of F-P cavity filled absorbent gas was derived from multi-beam interference theory. The transmission of the F-P cavity is the function of molecule absorption profile. With the increase of molecule absorption intensity,the width of the transmitted fringes increases. The measured line intensity and line width are consistent with HITRAN2004 database and theoretical calculation respectively.