CHINA'S best-known dissident, Wei Jingsheng, could be facing new criminal charges and a possible second prison term following his detention by the Beijing public security authorities. The official Chinese media announced yesterday that Wei was being investigated by the police for what it called ''suspected new criminal offences''. ''Wei is being interrogated and placed under surveillance by the public security authorities in accordance with the law because he violated the law on many occasions and is suspected of having committed new crimes while on parole and while being deprivedof his political rights,'' Xinhua (the New China News Agency) said. ''Surveillance'', according to the Chinese legal code is a form of house arrest whereby ''the judicial authorities will appoint a place for the criminal to live and use forceful measures to monitor his freedom of movement''. While a court official said Wei was being placed under surveillance ''where he lived'', his family said they had no information of his whereabouts. Xinhua did not specify what crimes Wei was suspected of committing but it is understood his outspoken criticism of the Government and the Communist Party and his contacts with other members of the dissident community and foreign journalists could providegrounds for new charges. Wei's original 15-year sentence was based on his alleged provision of ''military secrets'' to a foreign journalist and engaging in ''counter-revolutionary propaganda and incitement'', referring to his publications during the Democracy Wall movement of the late 1970s. Analysts said that Wei's statements since his release on September 14 last year have been just as critical as those made during the Democracy Wall period but point out that the political climate in Beijing has changed considerably since 1979. Most analysts believe Wei will be kept under surveillance at least until the fifth anniversary of the June 4 Beijing massacre so that he cannot take part in any new protests. ''What happens after that will very much depend on the political situation at the time,'' a diplomat said.