In1904-' 06 he undertook an expedition to Southwest China to search for plants suitable to be used as a garden plant in Great Britain. He engaged locals to assist him in collecting them. He gathered roots, tubers, seeds, and dried plants and invested a lot of time in describing his specimens. During this expedition all expedition members were murdered by Tïbetans. Forrest was the only one who managed to escape with help of local people. This probably had to do with the so called ‘British Campaign’ of 1903-' 04, an invasion by the British Indian army, in order to obtain a foothold in Tibet (for fear that Russia would interfere in Tibetan affairs), as was carried out in a similar way in other states around India, for instance in Afghanistan twenty years earlier. This invasion proved to be politically very unpopular at the home front in Britain and the effects on Tibet, except for victims and economic disruption, were minimal and changes did not last long.
At the end of Forrest's expedition many species were included in the collection of the Royal Botanic Garden in Edinburgh. Forrest undertook a total of seven expeditions to China and many plants he brought back were named after him (with the addition: Forrestii). At the end of his seventh expedition, he died of a heart-attack near the upper course of the Mekong, and was buried in the mountains of Yunnan.