The type 81 is a direct decedent of the Type 63/68. The type 63/68 was an amalgamation of everything the chinese liked out of the ak and the sks. They modified the gas system and bolt carrier out of an sks and adapted the magazine and rotating bolt out of the ak, while upgrading the mag to feature a bolt hold open device.
I'm just adding to what fenceline said, the type 81 was designed to replace the type 63/68, type 56 semi(SKS), type 56 assault rifle (AK47),
a intermediate rifle system until the development of 5.8mm family of rifles, it's the base rifle of the subsequent 5.8mm Type 87(prototype) and 5.8mm QBZ03(in service).
the type 63/68 was meant to be a upgrade to type 56 SKS after the sino-india war, but due to the political back ground (culture revolution) at the time and weak manufacturing capabilities, the type 63 (upgraded SKS) was plagued with problems and was finally abandoned by the military, a plan to develop a new rifle was started but the progress was slow almost stopped due to the "10 year culture revolution".
therefore during the 1979 sino-vietnam war, most Chinese soldiers are still equipped with SKS rifles, the Chinese soon realised they needed their own class of assault rifles/LMG,
the plan for a new rifle was brought back and fast tracked, by 1981, the T81 rifle series was born.
The design team liked many aspects of the AK47(T56A), but disliked others, mainly the lack of accuracy..severe muzzle flip during full auto fire.. etc,...
main similarities and differences between T81 and AK47 are,
1.T81 and AK share similar working internals, bolt lock up, feeding, recoil system, etc
2.T81's improved accuracy of 4.5cm group at 100m
3.T81 weights 3.4Kg which is lighter than the Chinese made AK(4.03kg) and weights almost the same as a Russian AK47(3.47kg)
4.Matching reliability of Russian made counter part, 20000 rds (claimed)without major parts failure.
7.Major Parts interchangeability (appox 70%) between all variants of T81 including LMG.
all the information was extracted from a interview with the T81 head designer, quite a interesting read, but sadly it's all written in Chinese, I don't have the time to translate the whole thing lol...I might one day when the t81 finally get here.
anyone interested(and able to read Chinese) can find the interview here...
http://jczs.news.sina.com.cn/p/2006-06-28/0730379888.html