The sexual function of a man in his forties varies from person to person, if the living habits are good, no disease influence, generally can keep normal. If the living habits are not good, the sexual function at the age of forty is easy to decline. To determine how a man’s sexual function at the age of 40 is, it is necessary to determine the specific situation, not generalized, there are obvious individual differences. Usually, for usually good living habits, and no disease influence, generally sexual function can remain normal, but the physical quality compared to the young adults, may have declined. Secondly, if there are bad habits in normal life, such as staying up late, alcoholism, smoking, etc., when you reach the age of forty, the sexual function will be easy to obviously decline, and sexual dysfunction, such as erectile dysfunction, premature ejaculation and so on, may occur. To summarize, if men want their sexual function not to be affected too much at the age of forty, it is recommended to strengthen the management of daily life, regular work and rest and sex life, and correct bad habits. If there are sexual dysfunctions, they should go to the hospital in time and cooperate with the doctor for active treatment.