China is not opposed to Indian entry per se. China is simply adhering to rules sketched out by NSG to become a member. India follows some of them conventionally but stays clear from legally binding provisions.
According to The Diplomat:
Where the United States and other supporting members have called for India’s inclusion based on New Delhi’s non-proliferation track record and the U.S.-India civil nuclear accord, China has made the NPT signature (or lack thereof) its central argument to scuttle India’s entry. Beijing has claimed that a “compulsory” requirement for NSG membership is that “the NSG members must be signatories to the NPT.”
Chinese state run Global Times noted:
Since its foundation in 1975, all NSG members shall be NPT signatories. This has become the primary principle of the organization. Now India wants to be the first exception to join the NSG without signing the NPT. It is morally legitimate for China and other members to upset India's proposal in defense of principles...
...US backing adds the biggest impetus to India's ambition. By cozying up to India, Washington's India policy actually serves the purpose of containing China.
..China's action [blocking NSG inclusion of India] is based on international norms, but India's reaction seems to indicate that their national interests can override principles recognized by the world.
What's more important is that NSG's decision requires consensus from all parties. So it is failure of Indian diplomacy and India supporting bloc in NSG (comprising Stats of mercantile interests including USA, UK, France, Australia) to convince China and other dissenting members as well.
It is interesting to note that when France was granted membership to NSG it was not a signatory to NPT. (Source)