I am a 43 years old man. for over seven years i sometimes develop rash on my penis?

It's difficult to assess your rash w/o any description. However, should this skin problem occur sporadically and resolve by itself after a week or so and you can tell exactly when it's going to occur by certain tingling or itching prior to the rash, it might be herpes simplex. See your dermatologist when the rash is present and get evaluated by phys exam and lab tests and given treatment

If it is sometimes and goes away then it is not serious. but if it bothers you pl see a dermatologist when you have it- for without an exam it is hard to diagnose this.