we have 5days trip on the second week of September in shanghai

Short-sleeved shirt and thin coat can be taken along at that time. Actually, it happens to be mid-September when typhoons can bring heavy rain in their wake. Thus, once you decide to outing then, you'd better bring an umbrella. And the average temperature could be about 26 C. By the way, as for the frequently changeable weather then, it can be better to pay much attention to the weather forecast.