  Multipartite entanglement is a key resource for quantum computation. It is
expected theoretically that entanglement transition may happen for multipartite
random quantum states, however, which is still absent experimentally. Here, we
report the observation of entanglement transition quantified by negativity
using a fully connected 20-qubit superconducting processor. We implement
multi-layer pseudo-random circuits to generate pseudo-random pure states of 7
to 15 qubits. Then, we investigate negativity spectra of reduced density
matrices obtained by quantum state tomography for 6 qubits.Three different
phases can be identified by calculating logarithmic negativities based on the
negativity spectra. We observe the phase transitions by changing the sizes of
environment and subsystems. The randomness of our circuits can be also
characterized by quantifying the distance between the distribution of output
bit-string probabilities and Porter-Thomas distribution. Our simulator provides
a powerful tool to generate random states and understand the entanglement
structure for multipartite quantum systems.
