ingenuous
adjective formal uk/ɪnˈdʒen.ju.əs/ us/ɪnˈdʒen.ju.əs/
天真的;胸无城府的
honest, sincere, and trusting, sometimes in a way that seems silly
It has to be said it was rather ingenuous of him to ask a complete stranger to take care of his luggage. 他让一个完全陌生的人帮自己照看行李，这未免也太天真了。
Opposite: disingenuous