  Extremely variable quasars (EVQs) are a population of sources showing large
optical photometric variability revealed by time-domain surveys. The physical
origin of such extreme variability is yet unclear. In this first paper of a
series, we construct the largest-ever sample of 14,012 EVQs using photometric
data spanning over $>$ 15 years from SDSS and Pan-STARRS1. We divide them into
five sub-samples according to the relative brightness of an EVQ during SDSS
spectroscopic observation compared to the mean brightness from photometric
observations. Corresponding control samples of normal quasars are built with
matched redshift, bolometric luminosity and supermassive black hole mass. We
obtain the composite SDSS spectra of EVQs in various states and their
corresponding control samples. We find EVQs exhibit clearly bluer (redder) SDSS
spectra during bright (dim) states, consistent with the "bluer-when-brighter"
trend widely seen in normal quasars. We further find the line EWs of broad
MgII, CIV and OIII (but not broad Hb which is yet puzzling) gradually decrease
from dim state to bright state, similar to the so-called intrinsic Baldwin
effect commonly seen in normal AGNs. Meanwhile, EVQs have systematically larger
line EWs compared with the control samples. We also see that EVQs exhibit
subtle excess in the very broad line component compared with control samples.
Possible explanations for the discoveries are discussed. Our findings support
the hypothesis that EVQs are in the tail of a broad distribution of quasar
properties, but not a distinct population.
