There are many examples of modernism in Lu Xun's short story "The Diary of a Madman," which is sometimes translated as "A Madman's Diary".

Before we get to them, let's cite a definition of modernism so that we have some basis of what modernism means. According to the Poetry Foundation, modernism

evolved from the Romantic rejection of Enlightenment positivism and faith in reason. Modernist writers broke with Romantic pieties and clichés (such as the notion of the Sublime) and became self-consciously skeptical of language and its claims on coherence.

It looks like we are going away from conventional thinking and standard morals. We're deviating from typically great and beautiful stories. Where are we going? It looks like we’re going into the imperfections of society. We're exploring its lack of reason, its flaws, and its foulness.

Think about how Xun describes the people in his short story. Do they look like they came out of a Romantic painting? It'd be hard to make an argument to support that.

The children he sees are "ghastly pale," just like Mr. Chao. He also notices a mother spanking her son. The mother tells her son,

Little devil! I'd like to bite several mouthfuls out of you to work off my feelings!

I think we can comfortably state that these are not idyllic, dreamy depictions of humans at their best.

You might also want to think about how Xun's story links to a rejection of reason. Think about his skepticism of Old Chen. Consider his fear of being eaten. How do these worries and anxieties seem unreasonable in a conventional way? Conversely, how do these fears seem reasonable in a modernist sense? How do they align with a modernist attitude toward a horrid, fragmented, decaying society?