It is possible to travel between the Chiang Rai and Kunming using Road 3Asia or R3A. This route is part of the GMS North-South Economic Corridor linking this region. You can break the trip into as many sections as you like, depending on the time you have available. Starting from Chiang Rai, the distance to Kunming is about 1,200 kilometres.
The first leg from Chiang Rai to Chiang Khong is 113 kilometres. From Chiang Rai City Bus Station local buses go to Chiang Khong regularly (mostly every half hour) from 5.00 AM to 16.00 PM.There are many red buses to Chiang Khong. If you not want to sit for a long time on a bus just choose one that does not pass Thoeng district (it would take one more hour for that bus).
From here you cross the Mekong River into Laos passing through Bo Keo district, Wieng Phu Kah and Luang Namtha to the border town of Boten. The total distance is 250 kilometres. From Boten, you enter China’s Yunnan Region at Mohan. The trip will then take you through Meng La, Mengxing, Jinghong, Puwen, Mojiang and Yuan Jiang before reaching Kunming. From Mengla to Jinghong takes about three hours and the last leg from Jinghong to Kunming is 520 kilometres, or nine hours.
Be prepared to spend a long time on the road. If you would prefer a shorter trip, China Eastern Airlines can fly you from Chiang Mai to Kunming directly.If you decide to stick with the bus, there’s one that runs direct from HuayXai to Kunming. You can do this trip in two days as the bus stops over in Luang Namtha and Jinghong on the way. Or you can take a bus to Jinghong, or even a local bus to Luang Namtha. For either of these options, you need to cross the Mekong River from Chiang Khong to Bokeo first, and spend a night in Bokeo to prepare to catch a bus the next morning.
You can also spend a night in Jinghong then take a bus to Kunming the next day. A direct service also runs between Bokeo and Kunming, but it’s a very long journey. I would advise breaking the journey at Luang Namtha in Laos. The small town is only two hours ride from the Chinese border. Jinghong is a good place to stop in Yunnan. You could also save a bit of time and the dizzy trip up and down the mountains by flying from Jinghong to Kunming.
Another option (less recommended) is to take a boat from Chiang Saen, hitching a ride on cargo boats from Chiang Saen up to China which would spend some 3 or 4 days on the river, and you have to hitch a ride on two different boats, spending a day or two anchored at some Burmese port waiting for a ride into China. There are two potential problems firstly, I’m not sure if the authorities will let you on the cargo boats, and secondly I’m not sure if the cargo boats will be willing to take you.