Monoenergetic electron bunches can be generated by the interaction between the ultra-intense laser and underdense plasma on the mm scale. In the experiment conducted on the SILEX-Ⅰ, by the interaction between an ultra-intense femtosecond laser pulse and 2.7 mm supersonic He gas jet, a 58 MeV quasi-monoenergetic electron beam with 15.5% energy spread and 15 mrad beam divergence is produced. The total charge of the electron beam is about 15.4 nC when the laser power is 70 TW. In this paper the experimental conditions, the method and main results are presented.