topped up the engine oil abt 3 months ago,at that time,everything seemed to be alright, and I didnt do any long journey these days, today when I opened the lid where I top up the engine oil, found lotof yellow grease mixed with oil,I dont knw wt that is,wt shall I do?

you might have an engine coolant leak. have it checked quickly.
also where is the yellow grease? if on filler cap, can be condensation mixed with the engine oil as the temperature on filler cap is much lower than inside the engine. this normaly happens if you drive a short journey and the weather is cool.

Thanks for the answer,it is on the filler cap and most of time I drive short journey,but the weather is not cool now, does it matter?

it should be fine.
as compared with the temperature inside engine, the filler cap's is much lower even when it is warm.
if i were you, i won't worry about it too much. in the mean time, check your coolant level and if you recognise the level drops quickly, have you car checked immediately.