Negative covariance is OK. It means that higher/lower results from one variable will happen at the same time as lower/higher results from the other variable. Only in the diagonal of the covariance matrix is always positive.

谢谢您的回答 自己英语不好 看起来虽然有点困难  但至少知道没有错误了

協方差標準化之後，就是相關係數。相關係數是有負數的。對角線上是方差，方差永遠大於等於0。