Tsinghua University is a leading and one of the most reputed universities in China, along with Peking University, which are both situated in Beijing. Traditionally, Tsinghua University is famous for its science subjects, while Peking University is famous for its arts subjects, akin to Cambridge and Oxford respectively.
In terms of hierarchy, Peking University is usually ranked first and then comes Tsinghua University, but most people deem both of them equally strong overall. However, for law and their law schools, Peking University is usually considered the better of the two, although locals would consider getting into either one as being a great achievement already, compared to other law schools in the country. In short, Tsinghua is about equal with Peking with both being the best universities in China, but it is second to Peking for law.
In terms of the LLM degree, it depends on what you want to get out of it. It's not going to be much of an improvement for employability (in the UK), although unless if you intend to stay in China long-term for work, it will be great investment.