这是填充。从http://en.wikipedia.org/wiki/Base64:

理论上，解码不需要填充字符，因为 缺失的字节数可以从Base64的数目计算出来 位数。在某些实现中，填充字符是强制的， 而对另一些人则不使用。填充字符的一种情况 需要连接多个Base64编码的文件。

它在RFC 2045中定义为一个特殊的填充字符，如果在编码数据的末尾可用的比特数少于24位。

不。 将base64编码的字符串填充为长度为4个字符的倍数，以便能够正确解码。

等号(=)在某些形式的base64编码中用作填充。关于base64的维基百科文章有所有的细节。

=是填充字符。如果输入流的长度不是3的倍数，则填充字符将被添加。这是解码器所要求的:如果没有填充，最后一个字节将有一个不正确的零位数。

更好更深入的解释在这里:https://base64tool.com/detect-whether-provided-string-is-base64-or-not/

The equals or double equals serves as padding. It's a stupid concept defined in RFC2045 and it is actually superfluous. Any decend parser can encode and decode a base64 string without knowing about padding by just counting up the number of characters and filling in the rest if size isn't dividable by 3 or 4 respectively. This actually leads to difficulties every now and then, because some parsers expect padding while others blatantly ignore it. My MPU base64 decoder for example needs padding, but it receives a non-padded base64 string over the network. This leads to erronous parsing and I had to account for it myself.