1984 was not
based on Stalin's dictatorship. There seems to be a common misconception among too many people who've heard this parroted, erroneously assuming that because Orwell was strongly wary of, and frequently wrote about,
totalitarianism, this must mean he was writing about Stalin specifically. He was not. In fact,
1984 was written just as much about the UK and USA as it was totalitarian fascism and Stalinism. This is borne out in his letters.
1984 was focused on what Orwell believed might happen to the world if the atomic showdown was inconclusive (a rough paraphrase of one of his remarks in a letter). It was written as a tale of what would happen when the West—specifically the UK and USA—would find themselves challenged and take the attractive slide away from democratically operated [socialist] society toward strengthening and worshipping millionaires and "petty fuhrers", leading to outright totalitarianism that offered stability, predictability, and controlled externalities, all of which would provide simpler, thoughtless, controlled selves.
For more directly from Orwell in one of his letters considered to foreshadow 1984's thesis, see here:
https://books.google.com/books?id=WXGbfAHYecwC&pg=PA232&lpg=...