From the end of spring to the beginning of summer (April to June) and autumn are the best seasons to visit Paris.
Spring（Average Temperature 10.5℃-19℃）
This is a flower blooming season which allows you to enjoy a pleasant trip. People drink coffee or beers in the terrace because it's warm and sunny after a long winter. But please still remember to bring a light coat in case of cold wind.
Summer（Average Temperature 15.5°-24.4°）
People around the world flood into Paris for a beautiful summer. The weather is generally lovely but sometimes can reach 32 °C . It doesn't sound so terrible, but when it comes to no air conditioner, inevitably there gonna be some sweaty nights. Seine River is a good place to get tanned because every year there will be a artificial beach called Paris-Plages( Bikini time!). You can go there and enjoy a cold drink and sunshine.
Don't miss the firework on every 14th July for France National Day.
Autumn（Average Temperature 9.2℃-15.8℃）
Autumn has mild days and fresh nights but are changing and unstable. Don't be too surprised if there is some warm or cool weather , because it frequently occurs. So, bring shorts and some nice warm outfits as well.
Winter（Average Temperature 2.5°-6.9°）
In winter, sunshine is scarce; days are cold but generally above freezing with temperatures around 7 °C (45 °F). Light night frosts are however quite common, but the temperature will dip below −5 °C (23 °F) for only a few days a year. Snow falls every year, but rarely stays on the ground. The city sometimes sees light snow or flurries with or without accumulation.
Information from https://en.wikipedia.org/wiki/Paris#Climate