my left shoulder a lit bit numb after change handlebar according to bike fitting
 

Recently I made bike fitting by shimano bike fitting system.my shoulder width is 400mm,my original bike handlebar width is 420 ,so everytime I cycle over 40km-200km,when I got home or on the last tens of km my shoulder is numb.but after change to 400mm handlebar ,it's a little better,but only left shoulder is a little numb.
so I just want to know it's because the handlebar angle or shift lever angle problem or I should use 380mm handlebar?

I don't know what a bike fitting by shimano bike fitting system is....numbness can be caused by a multitude of things related to bike fit (beyond just bar width).

so how to find the reason and resolve it.only left shoulder ,numb a little ,not too much.

Bar width usually doesn't make much difference. Numb, IME, means a pinched nerve or pinched blood vessel. Changing your fit caused this, so change your fit again.

I usually recommend getting the weight off your hands by moving your saddle back until you can briefly lift your hands from the bars while riding, without sliding forward on the saddle. Before moving your saddle, measure your saddle height and reset your saddle height after moving the saddle. After you're satisfied that your saddle position meats these criteria, next set your reach so that, with your hands on the hoods and elbows slightly bent and your back as straight as you can get it, your upper arms make a 90° angle with your torso. That'll probably mean changing stem lengths.

Make those changes, go ride, and see how that works for you.

thank you for your advice.so only one left side shoulder pain or numb ,is that related to the handlebar and shift lever angle?

Who knows? Your shoulders are obviously different in some way. I'm suggesting you make some changes, putting yourself into a normal road bike fit. If that's the identical fit your fitter has you in, obviously don't bother. But if it's not, give it a try.

Shift lever angle? You mean the location of your brifters? Set them up so they look like this: https://www.bikeforums.net/19145009-post3.html The levers are approximately vertical and the tops of the hoods form an extension of the bar's ramps, continuing forward in a more or less straight line.

Was the fit a one time deal? They don't let you come back and ask about things that aren't working for you?

not one time deal,I can come back and try.
but before I come back ,I am building a new road bike,so I want to finish it and then go fitting again.