How to scan strings from keyboard much clever?

"but when scanning the next string ,the buffer for saving pre_string will be changed"

So don't change it, then!

Have a buffer to receive data from the keyboard, and a separate buffer for data to display; you "scanning" stuff should only ever read from the input buffer.

My program structure as following:

static unsigned char idata sKeyBuf[15]= {0};
static unsigned char idata sScanBuf[10] = {0};
static unsigned char idata cForm = 0,cPrePos = 0;
unsigned char *ptr;

void main()
{
	...
	sprintf(sKeyBuf,"%s","12345678");

	while(1)
	{
		...
		cForm = StateMachine(key); //Scanning the functional key for switiching LCD PAGE
		ptr = ScanKeyboard(Key); //getting the string from keyboard
		ptr = DispFamat(ptr,cForm); //format string for displaying
		if ((cForm / 10) == 2) //getting in one LCD page
		{

			if (cPrePos != cForm % 10)
			{

				switch(cPrePos)
				{
					case 0:PrintFunc(obv,Pos0,sKeybuf); //  disp  full string obverse at position0 only when disp position be changed ;
					case 1:PrintFunc(obv,Pos1,sKeybuf);
					...
					default:break;
				}
				sprintf(sKeybuf,"%s\0",ptr);//if disp position changed ,the previous string be saved in sKeyBuf;
				cPrePos = cForm % 10; // if disp position changed ,cPrePos changed.
			}

			switch(cForm%10)  //positioning where to display
			{
				case 0:PrintFunc(rev,Pos0,ptr); // disp  string  reverse at position0 every byte input ;
				case 1:PrintFunc(rev,Pos1,ptr);
				...
				default:break;
			}
			...

		}
		...
	}
}

I'm sorry, but I don't understand your question. Could you try to explain the problem more clearly?

One thought: do you really intend your switch case statements to fall through?