What will it happened when the capacitance is almost the same as parasitic capacitance when doing post-sim ?
post-sim? You probably mean: post-layout extraction and simulation.
If your sampling capacitor has a value similar to the parasitic capacitance then you have a very strange design. In a "normal" design I would say that 80% of the capacitance should be from the sampling capacitor itself and 20 % or less should be from the parasitics.
If your sampling capacitor has a value similar to the parasitic capacitance then you probably have made your transmission gates too large in relation to the sample and hold capacitance.
Parasitic capacitance isn't always an issue, if the sampling cap. is connected between signal and ground and a parasitic capacitance is in parallel with that, the total sampling capacitance will simply have a larger value.
For other parasitic capacitances there are some techniques to minimize their influence. Should you apply these? No, only if you already know that there will be a problem.
Designing a circuit while trying to compensate for everything and including all kinds of "tricks" will result in a clumsy design. The trick is to know your requirements so that you can make educated decisions about what will be a problem and what will not be a problem.