Why my sunny showing mill light on running and slow down?

I assume you mean the MIL, or Malfunction Indicator Light, AKA the Check Engine light.

That light is intended to warn you that the engine computer has detected a problem in the engine or emissions system. The computer can tell you more about what problem it's seeing, but that requires a scan tool to be hooked up to the OBD-II port and the computer scanned for Diagnostic Trouble Codes. Here in the US, most auto parts stores will scan your car for codes for free, but since the Sunny isn't sold here (at least, not under that name), I assume you're not in the US. You can, of course, pay a mechanic to retrieve your trouble code or codes for you, or you can purchase an ELM327 Bluetooth dongle online for under $5 USD, which will connect to a free app on your phone and allow you to scan the codes yourself (I personally use one called Torque, but there are many available).

Once you have the engine code(s), you're welcome to come back and add that information to your question and at that point we may be able to point you in the right direction regarding how to troubleshoot and/or repair the issue.

The fact that the car is, as you put it, "slow down" could mean the issue is fairly serious, so you'll want to get your codes scanned as soon as possible before you do anything else with the car. If by chance the MIL is blinking, absolutely do not drive it, as that tells you there's a major problem that could result in catastrophic engine damage if it's not addressed.