Hello Liu!
We wish you the warmest welcome to the soundfont community.
Unfortunately, what you have described is not possible. Most (if not all) soundfont programs increase the pitch to speed up the whole sample. Therefore, it takes a shorter amount of time to reach the end of the sample. If you wanted the sample speed to be constant, it would result in undesirable sound artifacts, which is not what you want.
I have noticed that in the soundfont file you have attached, you have set the loop setting incorrectly. It's necessary to change the "loop playback" setting to loop (second option), rather than one-shot (first option). Doing this will not make the sample speed constant, but it will make it so the sample playing time is indefinite, rather than "very short".
Useful note: If you want to improve your sound quality, consider using more than one sample.
Thanks,
sylvia