it was不可以缩写成it's。be的过去式was,were一般不可缩写,不可以缩写的原因是因为缩写的话会造成时态混淆。如:
It was National day yesterday.
昨天是国庆节。(不能将was缩写)
She was sad last night.
她昨晚很伤心。
it's,it is的常用口语形式;it has的常用口语形式,尤用于has用作助动词时。
It's the world's biggest computer company.
它是全球最大的计算机公司。
It's among the poorer countries of the world.
它是世界上的贫穷国家之一。