Xi voiced his support in a video message to a virtual meeting of the WHO's decision-making body, the World Health Assembly.
The assembled leaders are expected to discuss a resolution being presented by the European Union that calls for an independent evaluation of the WHO's performance under its director-general, Tedros Adhanom Ghebreyesus.
Criticism of the WHO has been led by U.S. President Donald Trump.
Names on a draft resolution seen by Reuters show support from at least 116 of the 194 countries in the WHO.
President Xi also defended his country's own handling of the pandemic and pledged $2 billion over two years to help with the COVID-19 response.