Can I use iPhone fast charging and normal charging alternately?

Of course, it can be used alternately!

Today's iPhone not only supports ordinary 5v1a with USB to lighting cable for ordinary slow charging, but also supports 20 / 18W with type-C to lighting cable for fast charging, and also supports wireless charging.

The life of a lithium battery is only strongly related to the number of times it is charged and discharged. In other words, it is more related to your usage habits. These three charging methods can be switched at will, and will not cause much damage to the battery. If they really have such a big impact, I'm afraid that all the iPhone accessories will have to be tied up.

At present, all new iPhones will not be equipped with such a full range of accessories. For example, they clearly support fast charging, but they are equipped with an ordinary charging head and cable. They clearly support wireless charging, but you have to buy them yourself to make a profit. Just look at the price of iPhone accessories, which can cost hundreds of yuan
Even this suit is not cheap

So have used the original, fast charge slow charge on the random choice, which is convenient to use.