打草惊蛇 beat the grass and startle the snake
Explanation
比喻做法不谨慎，反而使对方有所戒备。
It is used to describe that careless actions only put the enemy on guard.
Origin Story
南唐时期，当涂县令王鲁贪财好色，鱼肉百姓，他的属下也纷纷效仿，贪污受贿。一天，百姓们联名上书，告发主簿贪赃枉法。王鲁知道后，并没有处理主簿，反而在状子上批示：“汝虽打草，吾已惊蛇。”意思是说，你们虽然告发了他，但我已经有所警惕了。这件事很快传遍了当涂县，百姓们意识到王鲁已经有所防范，便不敢再轻易地揭发他的罪行了。王鲁最终还是逍遥法外，继续为非作歹。
During the Southern Tang Dynasty, the magistrate Wang Lu of Dangtu was a corrupt official who oppressed the people. His subordinates followed suit, taking bribes. One day, the citizens jointly submitted a petition, accusing the chief bookkeeper of corruption and malfeasance. Wang Lu, instead of punishing the bookkeeper, wrote on the petition: “Though you strike the grass, I have already frightened the snake.” This meant that he was already on guard. The news quickly spread throughout Dangtu County. The citizens realized that Wang Lu was already suspicious, and dared not easily expose his crimes any longer. Wang Lu ultimately got away with it, continuing his misdeeds.
Usage
常用于否定句，指因行动不谨慎而使对方有所警觉。
Often used in negative sentences to express that one has put the enemy on guard by careless actions.
Examples
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警方这次行动打草惊蛇，让犯罪团伙提前转移了赃款。
jǐngfāng zhè cì xíngdòng dǎ cǎo jīng shé, ràng fànzuì tuán huǒ tíqián zhuǎnyí le zāng kuǎn
The police's operation this time alerted the criminal gang, causing them to transfer the stolen money ahead of time.
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为了避免打草惊蛇，警方决定秘密行动。
wèile bìmiǎn dǎ cǎo jīng shé, jǐngfāng juédìng mìmì xíngdòng
To avoid alarming the criminals, the police decided on a secret operation.
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这次行动由于准备不足，打草惊蛇，最终没能抓捕到罪犯。
zhè cì xíngdòng yóuyú zhǔnbèi bù zú, dǎ cǎo jīng shé, zuìzhōng méi néng zhuā bǔ dào zuìfàn
This operation failed due to inadequate preparation, and alarmed the criminals, ultimately failing to capture them.