An approach of producing recombinant trimers that mimic native HIV-1 envelope trimers is developed. A recombinant protein forming the recombinant trimers encompasses a recombinant HIV-1 gp140 fused to a tag through a linker at C-terminus of the recombinant HIV-1 gp140. The linker is sufficiently long so that the tag is accessible for binding by a binding molecule bound on a solid matrix. After expressed in a cell, the recombinant protein is secreted into the culture medium and assembles into recombinant trimers therein. The recombinant trimers may be directly purified from the culture medium. Cleaved and uncleaved trimers from different clade viruses are produced.