Thubten Wangchen, a member of the exiled Tibetan parliament, said Prime Minister Mariano Rajoy's government had moved to limit the use of "universal jurisdiction", which allows judges to try certain cases of human rights abuses committed in other countries, only to appease China.
A Spanish judge on Monday sought international arrest warrants for former Chinese president Jiang Zemin and four other top Chinese officials as part of a probe opened into alleged genocide in Tibet under this doctrine.
Thubten, a Spanish citizen who was born in Tibet in 1954 but exiled with his family when he was a child, brought the case in a Spanish court in 2006 along with two Tibetan support groups.
China, a significant economic partner of Spain, reacted angrily to the judge's move, saying it was "strongly dissatisfied".
Thubten said Spain's ruling conservative Popular Party "was listening and supporting the Chinese government and this is why it is reforming the law."
"There is no other reason. The Chinese government is putting a great deal of pressure on Rajoy's government and therefore poor Rajoy has to take note and obey China, there is no other choice," Thubten told AFP.
Under a bill introduced last month by the Popular Party, judges will be able to investigate crimes against humanity, war crimes and genocide only if the suspect is a Spanish national, a foreigner living in Spain or a foreigner in Spain whose extradition has been denied.
Spanish lawmakers will vote late on Tuesday whether to put the draft bill to a debate and vote.
The Popular Party has a comfortable majority in the assembly and the bill is assured to pass.
"If Spanish government changes the law at the request of China then that means it is China which is in charge in Spain. If the law changes it would be shameful for the Chinese government," Thubten said after visiting the Spanish parliament during its preliminary debate on the draft law.
"The former president of China did criminal deeds and is responsible for genocide in Tibet and now he is facing the bill and he has to pay that bill. It is normal," he added.
China considers Tibet an integral part of its territory, which it has ruled since 1951, a year after invading the Himalayan region.