is it bad if i use fruit juice in my smoothies?

If you think healthy, fruit juice is the worst of things you can ever get.

It is entirely sugar water. To make smoothie, since you are making it, blend the entire fruit or fruits with the smoothie. Nothing difficult, most you chop them first. That is healthier in magnitudes.

If juice is not 100% fruit its okay just know a smoothy can be 900 calories if too much fruit !

I made smoothies every morning. I try to keep the sugar level low, so I don't use fruit juice. I instead use rice milk or almond milk. I add a lot of vegetables (beets, carrots, celery, cucumber, spinach), some raw ginger, half of an avocado (though it's not necessary), an apple, and a handful of some other fruit, usually frozen strawberries, blueberries, or raspberries. I also add a spoonful of chia sees and psyllium husks.

Then I say relax, eat what tastes good and is made of real food and enjoy it. If you want to know if something has sugar added, look at the ingredients list, not the nutrition facts. Good food is a good thing.

It depends what your goals are and the purpose of your smoothies. Are you trying to lose weight, gain weight, looking for convenience?

I'm curious about why you think it might be bad? I use fruit juice in mine all the time.