Be going to do sth.表一般将来时，to do 是动词不定式；be doing sth.表现在进行时。

doing是现在分词。Be going to do sth.这是一般将来时态的构成。这个结构中的be有时态、人称和数的变化。如：I am going to be a doctor when I grow up.Tom said that he was going to help his mother with the housework.be doing sth.是进行时态的基本构成。在这个构成中，be是助动词，有时态、人称和数的变化， doing是现在分词。现在进行时的构成：am/is/are+现在分词They are playing basketball.过去进行时的构成：was/were+现在分词We were watching TV at nine last night.……