manifest.json

{
	"description" : "test description",
	"name" : "test name",
	"version" : "1.0",
	"content_scripts": [
		{
			"matches": ["*://*/*"],
			"js": ["myscript.js"],
			"run_at": "document_end"
		}
	],
	"permissions": ["clipboardWrite"],
	"browser_action": {
		"default_title": "test title",
		"default_icon": "test-0.png"
	},
	"manifest_version": 2
}

myscript.js

chrome.extension.sendMessage({}, function(response) {
    var readyStateCheckInterval = setInterval(function() {
    if (document.readyState === "complete") {
        clearInterval(readyStateCheckInterval);
        // ----------------------------------------------------------
        // This part of the script triggers when page is done loading
        console.log("Hello. This message was sent from scripts/inject.js");
        alert('complete');
        copyTextToClipboard( document.body.innerHTML );
        // ----------------------------------------------------------

    }
    }, 10);
});

// 需要在 manifest.json 中设置 clipboardWrite 权限
function copyTextToClipboard(text) {
  var copyFrom = document.createElement("textarea");
  copyFrom.textContent = text;
  var body = document.getElementsByTagName('body')[0];
  body.appendChild(copyFrom);
  copyFrom.select();
  document.execCommand('copy');
  body.removeChild(copyFrom);
}