Siberian tigers are the largest of all cats. Their current territory is southern Russia bordering northern China and North Korea. There are thought to be about 500-600 Siberian tigers living in the wild - but China only accounts for 50-60.
Siberian tigers are orange, not white. White tigers are the result of recessive genes and are rare in the wild.
Human encroachment on Siberian tigers' habitat is the main reason for their lessening numbers but poaching is also a real problem in China as tiger skins are worth a great deal on the black market and tiger bones are especially valuable in traditional Chinese medicine. The Chinese government protects tigers but poaching does still happen.
You very likely won't see a Siberian tiger in the wild in China. The best place to see them is in Harbin's Siberian Tiger Park. While you may take issue with how the park is run (many see it more as a mass tourist attraction than a serious conservation center), it is the only place to see these magnificent animals up close.