如何在activity中获取fragment的控件，然后修改控件的内容，

你的Activity里面注册过fragment吗？
像这样========》

private void InitViewPager() {
mPager = (ViewPager) findViewById(R.id.vPager);
fragmentsList = new ArrayList();

Fragment activityfragment = TestFragment.newInstance("Hello Activity.");
Fragment groupFragment = TestFragment.newInstance("Hello Group.");
Fragment friendsFragment=TestFragment.newInstance("Hello Friends.");
Fragment chatFragment=TestFragment.newInstance("Hello Chat.");

fragmentsList.add(activityfragment);
fragmentsList.add(groupFragment);
fragmentsList.add(friendsFragment);
fragmentsList.add(chatFragment);

mPager.setAdapter(new MyFragmentPagerAdapter(getSupportFragmentManager(), fragmentsList));
mPager.setCurrentItem(0);
mPager.setOnPageChangeListener(new MyOnPageChangeListener());
}