The chitosanase gene (btbchito) of Bacillus thuringiensis BMB171 was cloned and heterologously expressed in the yeast Pichia pastoris. After purification, about 300 mg of recombinant chitosanase was obtained from the 1-1 culture medium with a specific activity of 240 units/mg. Results determined by the combined use of thin layer chromatography (TLC) and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry (MS) showed that the chitooligosaccharides (COSs) obtained by chitosan (N-deacetylated by 70%, 80%, and 90%) hydrolysis by rBTBCHITO were comprised of oligomers, with degrees of polymerization (DP) mainly ranging from trimers to heptamers; high molecular weight chitopentaose, chitohexaose, and chitoheptaose were also produced. Hydrolysis products was also deduced using MS since the COSs (n) are complex oligosaccharides with various acetyl groups from one to two, so the non-acetyl COSs (GlcN)n and COSs with more acetyls (> 2) were not detected. The employment of this method in the production of high molecular weight COSs may be useful for various industrial and biological applications, and the activity of chitosanase has great significance in research and other applications.