"下意识心理活动" can be translated into English as "subconscious mental activity" or "unconscious psychological processes".
"Subconscious mental activity" emphasizes the mental processes that occur below the level of conscious awareness.
"Unconscious psychological processes" also conveys the idea of mental activities that are not within conscious control or awareness.
Both expressions are commonly used in psychology and related fields to describe thoughts, feelings, or behaviors that occur without our direct awareness or control.