这里的watching movies 是finds的宾语,frustrating是宾语补足语.
换一种方式你就知道这句话怎么得来的：
watching movies is frustrating.是个句子,其中watching movies 是主语, is 是系动词frustrating是表语这个句子可以等于It is frustrating to watch movies .
he finds (that) watching movies is frustrating. 这时 watching movies is frustrating是一个宾语从句.而find 可以构成句型： find+宾语+宾补.这样把原来宾语从句中的is去掉,就形成he finds watching movies frustrating.= he finds (that) watching movies is frustrating=he finds it frustrating to watch movies= he finds (that) it is frustrating to watch movies