Abstract: The Inconel 690 alloy (aging state) for steam generator heat transfer tube was subjected to hydrostatic test at different pressures (35, 55 MPa). The deformation behavior of the test alloy during hydrostatic test was studied. The initiation of each slip system in grains during deformation was revealed by lattice rotation analysis. The results show that compared with that before the hydrostatic test, after hydrostatic test at 55 MPa, parallel slip bands and dislocation walls appeared in grains of test alloy, the average grain size decreased from 82 μm to approximately 65 μm, and the proportion of annealing twins did not change significantly. The average dislocation density increased from 4.68×10¹³ m⁻² before the hydrostatic test to 1.19×10¹⁴ m⁻² after 55 MPa hydrostatic test. The deformation of the alloy was mainly driven by dislocation slip. Increasing pressures could increase equivalent plastic strains and the proportion of each slip system in grains. The most initiated slip system in grains of the test alloy was ($\overline{1}$11)$\overline{1}\overline{1}$0 at 35 MPa pressure, while the most initiated slip system changed to (111)1$\overline{1}$0 at 55 MPa. Increasing pressures would trigger more dislocation slips, and the dominant slip system also changed.