Problem solved. Thanks a lot.
The dynamic writer (Schema Sources: Schema from Schema Feature) would not write the first feature into the destination dataset, if it has an attribute called "fme_schema_handling" with the value "schema_only", which the schema feature output from the <Schema> port contains.
I don't think you need to use the FeatureMerger. You can just send the schema feature and data features to the writer feature type separately.