Effective　removal　of　acetaldehyde　and　propionaldehyde　from　propylene　oxide　is　important　for　polyether　industry　but　remains　a　significant　challenge　due　to　their　similar　molecular　sizes　and　relatively　close　volatilities.　Herein,　we　studied　flexible　molecular　materials,　perethylated　pillar[n]arene　crystals　(n　=　5,　6),　which　can　be　used　to　separate　acetaldehyde　and　propionaldehyde　in　propylene　oxide.　Through　a　space　size　matching　effect,　perethylated　pillar[5]arene　demonstrates　excellent　adsorption　selectivity　of　95　%　and　100　%　for　aldehydes　in　ternary　mixture　and　binary　mixture　of　propylene　oxide,　acetaldehyde　and　propionaldehyde,　respectively.　These　impressive　selective　adsorption　is　achieved　based　on　their　differential　host-guest　interactions.　Density　functional　theory　calculations　provide　a　comprehensive　explanation　of　the　adsorption　process,　revealing　that　the　adsorption　mechanisms　of　perethylated　pillar[5]arene　for　acetaldehyde　and　propionaldehyde　primarily　involve　C-H　&　sdot;&　sdot;&　sdot;O　&　sdot;&　sdot;&　sdot;　O　hydrogen　bonding　and　C-H　&　sdot;&　sdot;&　sdot;pi　&　sdot;&　sdot;&　sdot;　pi　interactions.　This　study　also　provides　a　novel　method　for　achieving　one-step　purification　of　propylene　oxide　in　industrial　applications.