The invention is directed to the field of natural medicine, specifically to alkaloid extracts of Huperzia serrata (Thunb.) Trev. A Huperzia serrata (Thunb.) Trev. composition and the method for preparing it are provided. The Huperzia serrata (Thunb.) Trev. composition comprises Huperzine A and Huperzine B, wherein the total content of the said Huperzine A and Huperzine B is 50-99 wt % of total alkaloids and the weight ratio of the said Huperzine A and Huperzine B is 1:0.5-5. The Huperzia serrata (Thunb.) Trev. composition of the invention has good therapeutic effect and improved safety.