Foreign Ministry spokesman Ma Zhaoxu said China had "indisputable sovereignty" over the island, in the Spratlys chain.
A group of Philippine congressmen had flown to Pagasa island on Wednesday and declared it Philippine territory.
The Spratly Islands are also claimed by Vietnam and Taiwan.
The Spratly dispute is part of a larger territorial row, which also involves Brunei and Malaysia.
The regional grouping Asean (Association of South East Asian Nations) is currently meeting in Bali, where talks have focused on framing guidelines for resolving the dispute.
Tension has been growing this year, with China making a series of tough statements warning off Vietnam and the Philippines.
"China has indisputable sovereignty over the South China Sea islands and their surrounding waters," said Mr Ma.
"The move of the Philippine side seriously infringed China's territorial sovereignty." (more)