Library for Baseband (lbb) is a self-developed open source library for reading and analyzing VLBI (Very Long Baseline Interferometry) baseband data, which is mainly used for reading and analyzing terminal baseband data in VLBI observations.This software library can analyze different data formats based on the algorithm, and also provide a variety of Application Programming Interface (API) functions for the user.At present, lbb library had been successfully applied to the hardware correlator for Changé lunar project,and the data observation of geodetic VLBI.This paper introduces the design, function and usage of lbb in detail.