This dataset contains the numerical data underlying the six physics figures (Figs. 2–7) presented in the paper “Studies of directed flow with event plane method in the HIRFL-CSR External-target Experiment”. The data are intended to allow readers to reproduce the published plots and to facilitate further comparison and reuse.The underlying events are 500 MeV/u 238U+238U collisions at sNN≈2.1 GeV generated with the JAM transport model(https://gitlab.com/transportmodel/jam2) and propagated through the CEE detector using the CFS fast simulation framework(https://gitee.com/CEESM/CeeRoot), including the TPC, MWDC and ZDC sub-detectors as described in the paper. The final-state observables stored in this dataset are the quantities used to draw: (i) the proton acceptance in TPC, MWDC and ZDC (angular and kinematic coverage); (ii) the first-order event-plane angle distributions before and after recentering, position-weight and shift corrections; (iii) the first-order event-plane resolution as a function of impact parameter for MWDC and ZDC; (iv) the rapidity dependence of proton directed flow v1v_{1}v1​ obtained from the true reaction plane, from the MWDC and from the ZDC event planes; (v) the comparison of v1(y)v_{1}(y)v1​(y) slopes extracted from different event planes; and (vi) the comparison of proton v1(y)v_{1}(y)v1​(y) for two detector-efficiency settings (100% and 90%).All information is stored in a small set of ROOT (CERN) files (*.root) containing histograms and graph objects corresponding to each physics figure.