SHENZHEN—China’s economy is slowing faster than expected as Beijing this week heads into a crucial new round of negotiations with the U.S. over trade.

For the past couple of months, senior Chinese officials have sought to play down the impact of U.S.-China trade tensions on the world’s second-largest economy, telling the public the conflict is doing little more than hurting the nation’s stock indexes, making it a good time to buy China again.

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