I have completed my graduation with7.18 CGPA,inter with70.8%,high school with53.67%,am I eligible for CAT entrance exam2017

Yes you are eligible for CAT as min percentage they require is 50% in any academic degree. But as you know the weightage of these marks will carry forward in your aggregate for your first call for interview and also for final aggregate list.

The weightage for 10th 12th and graduation varies for each college. Having a less percentage will be a problem for you so to cover it up you need to aim for a very high percentile in CAT exam, atleast 98+ in order to end up enrolling in a good college.

PS: These percentages also play a vital role even after your admission during internship placements.

Anyway in short you are eligible for CAT exam and aim to end up with a high percentile.