can someone explain to me? it is right that I saw bus is coming means I was standing and a bus is coming on the road?
I saw a bus coming and I saw that a bus was coming are grammatical ways to say that, in the past, you saw that, in the past, a bus was approaching you but had not yet arrived at your location. You could be sitting or lying on the sidewalk when you made that observation. "See" does not imply your bodily orientation when you make a visual observation.
If you want to indicate your bodily orientation you should say I was standing when I saw the bus coming at me on the road.
Your first attempt is wrong in two respects. First, you need an indefinite article before "bus." Second. the sequence of tenses makes no sense: how could you have seen in the past something that is occurring in the present.
Your second attempt is perfectly grammatical, but technically your second example does not state the same thing as your first: a bus need not always drive on a road nor does seeing imply standing.
There are other grammatical problems with both statements, but let's just focus on the word common to both sentences "saw" and the difference between "is" and "was".
"Saw" is the past tense of see.
"is" is present tense.
"was" is past tense.
You should not mix tenses in the same sentence, so...
I saw (a) bus is coming
is wrong because you talk about the bus coming in the present tense when you saw it in the past.
I saw (a) bus was coming
is acceptable and you might hear it said, but it is still not 100% correct because there is no need to state the past tense twice.
As you have already established the past tense by the word "saw", you could simply say:
I saw a bus coming.