I'm an office lady, works very long a day in my office. I often sat at least 9 hours a day in the office. It's hard to keep balance in work and sports. What do you often do to keep health in office?

First, be sure you chair is the right height for your height. With your feet flat on the floor, there should be a small gap between the underside of your legs and the chair.
Second, don't lean forward. Try to stay straight upright, and keep your back straight. Most of us hunch over.
Third, don't move one foot under the chair, or lean to one side.
Fourth, your arm should rest comfortably on the desk, without lifting your shoulder. If you can't, then your desk is too tall. Of course getting an employer to lower a desk is very difficult. But you can't raise your chair unless you have a footstool!
(I have to constantly remind myself.)

Get up and walk around even for one minute at least twice an hour. Stretch, wiggle, rotate your torso, anything you can do, even if it's standing right beside your desk.

In addition to what joy has suggested, check various videos online for exercising while seated. Do get up frequently and move about. Make some of your regular movements more productive... park farther away (safely) and walk to and from your work place, use stairs if available, stretch when you reach for something, etc. I know a few people who have switched their office chair for an exercise ball with a stand for back support.

Here office staff in many places take breaks every 2 hours or so, and do 10 minutes of exercise. Also short walks at lunch, get up a little early and do exercise in morning and every evening.

First, be sure you chair is the right height for your height. With your feet flat on the floor, there should be a small gap between the underside of your legs and the chair.
Second, don't lean forward. Try to stay straight upright, and keep your back straight. Most of us hunch over.
Third, don't move one foot under the chair, or lean to one side.
Fourth, your arm should rest comfortably on the desk, without lifting your shoulder. If you can't, then your desk is too tall. Of course getting an employer to lower a desk is very difficult. But you can't raise your chair unless you have a footstool!
(I have to constantly remind myself.)

Get up and walk around even for one minute at least twice an hour. Stretch, wiggle, rotate your torso, anything you can do, even if it's standing right beside your desk.

I always forget the second one, it's hard for me to do. What do you mean in the forth step?

Many of us short people have desks that are too high. When you are seated properly in a chair at the right height for your feet, the rest your arm on the desk. Your arm should not be lifted up at all, and you shouldn't have to hunch down either! We tend to usually raise it, and over the long term, that can create excruciating pain in your neck and shoulder.