What on Earth do you mean? First, the vast majority of China is on the internet, secondly there are non-internet methods of polling. Here's the source, and the relevant section for how they took the data:

The survey team set out to assess overall satisfaction levels with government among respondents from across the socioeconomic and geographic strata of China. “It is always a challenge to obtain a representative sample of the Chinese population, particularly from interior provinces,” said Turiel. “Our survey does not include migrant laborers, for example. But given the fact that the survey conducted in-person interviews with over 3,000 respondents per year in a purposive stratified sample, we are happy that the results include not just the coastal elites or large urban areas, but also poorer and less developed inland provinces.”

Emphasis mine.