#1
Hi, everyone,

My classical guitar which I bought for 60$ start to sound not so good after changing a string like last 3-4 month ago.

I bought it for half year and never change its orignal string and it has saddle remove due to aging.

My current string is thinner than the previous one and I had no ideas how important the string is to classical guitar. For my electric guitar, every brand string sound almost the same if it has the same gauge. 

What can I do to make sound a little better and does thick string produce a better sound? I know that classical guitar tend to sound millow but I'm not trying to make it sound like acoustic. Just want to make it more heavier.

Additionally, My guitar is Yamaha c150.
#2
The first thing to do is to replace the saddle. It may have been removed to lower the action but it's a wrong move.

Secondly, you say the new strings are "thinner". Now although classical nylon strings do come in different tensions the actual string diameters do not vary by very much. Are your strings traditional nylon ones?

Some pics would help.
#3
Gotta have a saddle, that's why it sounds bad.  It also will not intonate properly which sounds "bad".  The further up the fretboard you go the worse it gets.  If the saddle was removed because of too high action that would most likely be from someone putting steel strings on it and pulling up the bridge and/or bending the neck which does not have a truss bar.  
Replace all the strings to assure continuity.
#4
What if I add back the saddle? Will the high action prevent me from playing fast on higher fret? I usually try electric stuff/licks on my classical because getting electric guitar ready take sometime. And what about the lower fret? will it be harder to play? The current no saddle action is meet the standard measurement and if I add saddle, I guess that it will add more mm to it.
#5
Quote by sosxradar
What if I add back the saddle? Will the high action prevent me from playing fast on higher fret? I usually try electric stuff/licks on my classical because getting electric guitar ready take sometime. And what about the lower fret? will it be harder to play? The current no saddle action is meet the standard measurement and if I add saddle, I guess that it will add more mm to it.

Yes, replacing the saddle will raise the action. You can sand down the base of the saddle to reduce its height - you only need a mm of the saddle standing proud of the bridge. 
#7
Check that neck angle (same plane as the body?) and curvature (should be almost dead straight) with a loooong straight edge, then check first fret to saddle position.  3 - 4mm between bottom of the bottom E string to the top of the 12th fret should be OK for action.