I m creating a artwork in CMYK mode and i want a jpg in RGB form but colors art different in output. This problems is started from updating illustrator CC 2017.
It's hard to tell for sure without a picture, but what you're describing sounds like normal behavior, when converting CMYK to RGB.
The two color spaces are different from each other in several ways and the intended use of your design should be considered, when choosing your initial document settings.
I won't get into too much detail about the differences between the two here*, but know that they have slightly different gamuts (ranges of colors they can display).
See below for a visualization of the two:
As you can see, the CMYK and RGB gamuts overlap in many areas, but each color space has ranges of colors not available in the other. This is where your conversion issues are likely coming from.
If you are designing for screens, use RGB from the beginning
If you are designing for print, then use CMYK
*There are tons of resources available online describing the differences between the two color spaces. This post has some decent answers, to help you learn more about CMYK/RGB color spaces.