The　effects　of　oxide　aperture　shape　whose　size　is　small,　on　the　mode　characteristics　of　external　cavity　oxide-confined　photonic　crystal　vertical　cavity　surface　emitting　laser　(VCSEL)　is　studied　extensively　based　on　the　finite-difference　time-domain　algorithm.　A　new　three-dimensional　model　of　optical　field　in　photonic　crystal　VCSEL　is　built　and　the　influence　of　the　variation　in　aperture　shape　on　the　characteristics　of　far-field　and　spectrum　is　pointed　out.　It　is　found　that　the　modal　characteristics　of　PhC-VCSEL　are　influenced　by　the　shape　of　oxide　aperture,　the　influence　on　spectrum　feature　is　more　apparent.　It　can　be　explained　from　the　distribution　of　modal　field　by　the　fact　that　the　symmetrical　characteristic　of　prismatic　oxide　aperture　is　not　identical　to　that　of　high-order　mode.　But　the　influence　is　less　apparent　while　the　depth　of　photonic　crystal　hole　is　deeper.　The　reason　for　this　matter　is　illustrated.　The　results　can　be　used　to　guide　the　fabrication　of　external　cavity　oxide-confined　photonic　crystal　VCSELs.