Covalent organic frameworks (COFs) based membranes have demonstrated great potentials in molecular separation. Currently, most studies focus on developing continuous COF membranes. However, their low crystallinity compared with COF powders due to the more restricted synthesis conditions impedes their application in precise separation. Herein, we explored the assembly of highly crystalline COF powders with chitosan (CS) to construct precise-sieving membranes with high COFs loading. A post-carboxylated COFs (TATP-COOH) was synthesized, and a facile and scalable strategy was developed for constructing the assembled COF membranes (TATP-COOH@CS/TATP-COOH/CA) with TATP-COOH loading as high as 40 wt% (82 v%). Characterizations revealed TATP-COOH particles could interact with CS via H-bonding to form dense TATP-COOH@CS layers, and the TATP-COOH layer on the CA substrate was essential to obtain mechanically stable membranes. The prepared membranes exhibited precise rejection to Congo red, Coomassie brilliant blue (BB), Alcian blue and Direct blue 19 with 100% rejection and ultrafast permeance (>1350 L m⁻² h⁻¹ MPa⁻¹). It also showed superior precision in separating Methyl orange and BB mixed solution with only Methyl orange passing through. This study provides a significant concept and a facile strategy to the preparation of high precise-sieving COF membranes to take advantages of highly crystalline COFs.