BEIJING — For Ai Weiwei, the outspoken Chinese artist, the idea was irresistible.

Mr. Ai had become obsessed with a red scribble that appeared on a planning document for an art show next month in Yinchuan, a city in northwest China. He decided to build a large sculpture modeled on the line that he would call “Redline” — a playful rumination on the idea of censorship.

But this week, the artistic director of the Yinchuan Museum of Contemporary Art, Suchen Hsieh, sent Mr. Ai a cryptic message:

“Bose [Krishnamachari, Indian artist and curator of the biennale] and I invited you to participate in this year’s Yinchuan Biennale because we sincerely admire your artwork. But things change in this world. Even though your project is full of philosophical awareness, an artist’s prestige overshadows his work. The autumn wind is blowing around us. The museum has no choice but to rescind its invitation to you. It’s very unfortunate that the conditions don’t allow us to display your artwork. “This is the second time I must clasp my hands together and bow to you from afar. Please accept my deep apologies.”

Mr. Ai, who provided a copy of the message, was taken aback. “It was a very strange note,” he said in a telephone interview on Thursday from Berlin, where he lives.

In response, he posted a 173-word critique on Instagram, denouncing efforts to limit free expression. “Art is used merely as a decoration for political agendas in certain societies,” he wrote. He also posted on Twitter, referring to Ms. Hsieh as Xie Suzhen, using the Pinyin spelling of her name: